Load issue templates from repository

part of #18656 


## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] API support added
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !4981

Backport changes from gitlab-org/gitlab-ee!581

## What does this MR do?

Backports changes that were made in gitlab-org/gitlab-ee!581, to avoid potential merge conflicts in the future.

## What are the relevant issue numbers?

- Related to gitlab-org/gitlab-ee!581

## Does this MR meet the acceptance criteria?

## Tasks

- [ ]  !5824 Backport changes from EE!581 to CE
    - [x]  Implementation
        - [x]  ::ProtectedBranches::CreateService.new
        - [x]  Can't remove `load_protected_branches_gon_variables`
        - [x]  `has_many` with count enforced
        - [x]  Extract from access levels
        - [x]  project.protected_branches.create(params)
        - [x]  Improve "access_levels.first"
        - [x]  Fix tests
    - [x]  Fix build
    - [x]  Assign to Douwe
    - [ ]  Wait for review/merge

See merge request !5824

Added documentation on adding database indexes

## What does this MR do?

This MR adds a guide on when to add database indexes, how to approach this problem, etc

## Are there points in the code the reviewer needs to double check?

Spelling, grammar, etc

## Why was this MR needed?

Blindly adding indexes can be harmful to the database, and I recently had to remove 48-something unused indexes.

## What are the relevant issue numbers?

#20767

## Does this MR meet the acceptance criteria?

- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- Tests
  - [x] All builds are passing
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5771

- Use assertions in the vein of `merge_access_levels.map(&:access_level)`
  instead of `merge_access_levels.first.access_level`

Revert "Merge branch '19957-write-tests-for-adding-comments-for-different-line-types-in-diff' into 'master'"

This reverts merge request !5417

Add reference to product map.

## What does this MR do?
Mention the product map in the UI Guide and remove a line that says that the GitLab logo and user picture and in the sidebar.

## What are the relevant issue numbers?
19350

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)
Closes #19350

See merge request !5769

- `#find_users` depends on a project being loaded.

- Missed adding this in 2193ae22

- This is an optimization that was made in !581, and it needs to be
  backported to CE to avoid merge conflicts in the future.

- Previously created a protected branch manually, without using a
  factory.

1. `GitPushService` was still using `{merge,push}_access_level_attributes` instead
   of `{merge,push}_access_levels_attributes`.

2. The branches API creates access levels regardless of the state of the
   `developers_can_{push,merge}` parameters. This is in line with the UI,
   where Master access is the default for a new protected branch.

3. Use `after(:build)` to create access levels in the
   `protected_branches` factory, so that `factories_spec` passes. It
   only builds records, so we need to create access levels on `build` as
   well.

1. Previously, we were using `after_create` to create access levels.

2. At the time of protected branch creation, there are _no_ access
   levels present, which is invalid, and creation fails.

3. Fixed by setting access levels before the protected branch is created.

1. Change a few incorrect `access_level` to `access_levels.first` that
   were missed in e805a647.

2. `API::Entities` can iterate over all access levels instead of just
   the first one. This makes no difference to CE, and makes it more compatible
   with EE.

1. To improve EE compatibility.

1. This is in regard to the protected branches feature spec.

2. For example, if "Masters" is already selected, don't re-select
   "Masters" during the spec.

3. This is due to a bug in the frontend implementation, where selecting
   an already-selected access level _deselects_ it, which is something
   we don't need. I'll create a separate issue for this.

4. This hasn't turned up before, because we were manually creating
   missing access levels prior to e805a647. Now, we just use nested
   attributes, and missing access levels fail validation.

!581 has a lot of changes that would cause merge conflicts if not
properly backported to CE. This commit/MR serves as a better
foundation for gitlab-org/gitlab-ee!581.

= Changes =

1. Move from `has_one {merge,push}_access_level` to `has_many`, with the
   `length` of the association limited to `1`. This is _effectively_ a
   `has_one` association, but should cause less conflicts with EE, which
   is set to `has_many`. This has a number of related changes in the
   views, specs, and factories.

2. Make `gon` variable loading more consistent (with EE!581) in the
   `ProtectedBranchesController`. Also use `::` to prefix the
   `ProtectedBranches` services, because this is required in EE.

3. Extract a `ProtectedBranchAccess` concern from the two access level
   models. This concern only has a single `humanize` method here, but
   will have more methods in EE.

4. Add `form_errors` to the protected branches creation form. This is
   not strictly required for EE compatibility, but was an oversight
   nonetheless.

Upgrade httpclient gem from 2.7.0.1 to 2.8.2.

Fixes deprecation warnings from Ruby 2.3. Resolves #20950.

Changelog: https://github.com/nahi/httpclient/blob/b51d7a8bb78f71726b08fbda5abfb900d627569f/CHANGELOG.md#changes-in-282

cc: @stanhu 

See merge request !5816

Submit to Akismet Part 1 (Issues)

Related to #5932 #5573 gitlab-com/infrastructure#14

See merge request !5538

Add rspec feature tests for the diff notes feature.

## What are the relevant issue numbers?

Closes #19957.

See merge request !5417

Fixes deprecation warnings from Ruby 2.3.

Changelog: https://github.com/nahi/httpclient/blob/b51d7a8bb78f71726b08fbda5abfb900d627569f/CHANGELOG.md#changes-in-282

This fixes a long running tests due to changed Sidekiq state

## What does this MR do?
Fixes a ordering of tests problem, where GroupsController tests change global state and this affects IrkerService

cc @connorshea 

See merge request !5819

Allow to sort by project size on admin projects page

Fixes #20317

See merge request !5797