Revert the revert of Optimistic Locking

## What does this MR do?

It reverts the revert of optimistic lock https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5146 (and revert MR https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5245)

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/19871

Actually the code is already reviewed, except:
* Monkey patch `config/initializers/ar_monkey_patch.rb`
* We removed default values from migration `db/migrate/20160707104333_add_lock_to_issuables.rb`

See merge request !5623

Fix location of markdown help

Resolves #21208, and incorrect link to the markdown style guide

See merge request !5945

Tests for diff notes (without failures 🙏)

## What does this MR do?

Adds specs for diff notes.

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

We need to ensure that this doesn't break master again.

## Why was this MR needed?

## What are the relevant issue numbers?
Closes #20957
## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [ ] [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)

See merge request !5864

Fix for update 8.10-to-8.11.md doc.

## What does this MR do?

Fixed a typo in the docs to update Ruby.

See merge request !5941

Bump SimpleCov merge timeout to 365 days

## What does this MR do?

In case that CI pipeline fails because of intermittent errors in one of the jobs, when someone retries the only failing job located in the middle stage, the job that calculates compound coverage that is configured in later stage, and triggered after retrying job in the pipeline, will still return an accurate coverage value.

Closes #21144

See merge request !5932

Add Ruby 2.3 upgrade notes.

The 8.10 to 8.11 docs didn't mention Ruby 2.3, now they do.

See merge request !5940

Split Koding admin/user docs

Backend/Frontend MR https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5909

See merge request !5938

Documentation for Issue Boards

## What does this MR do?

Add documentation for issue boards.

## What are the relevant issue numbers?

- Issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/17907
- Backend MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5548
- Frontend MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5554

See merge request !5713

Change slash commands docs location



See merge request !5937

Fix typo in gitlab-workhorse header

See merge request !5891

Document protected branches new behavior



See merge request !5934