Deleting a user shouldn't delete associated issues.

Closes #12726

See merge request !7393

Use Namespace#full_path instead of #path where appropriate

See merge request !9495

Simplify variables validation in triggers API

See merge request !9506

Use correct GitLab Prometheus exporter name in docs

Closes #28619

See merge request !9503

Added application password instructions for Gmail/G suite accounts

See merge request !9486

Fix broken links in CI admin area docs

See merge request !9507

clarify Pages custom domain details

Closes #28577

See merge request !9457

Api remove owned groups

Closes #27937

See merge request !9505

Use persistent name identifier instead of transient in SAML2 documentation

Closes #28653

See merge request !9504

[ci skip]

# Conflicts:
#   doc/api/v3_to_v4.md

Return 202 with JSON body on async removals on V4 API

Closes #3874

See merge request !9449

- Add a `destroy_user` ability. This didn't exist before, and was implicit in
  other abilities (only admins could access the admin area, so only they could
  destroy all users; a user can only access their own account page, and so can
  destroy only themselves).

- Grant this ability to admins, and when the current user is trying to destroy
  themselves. Disallow destroying ghost users in all cases.

- Modify the `Users::DestroyService` to check this ability. Also check it in
  views to decide whether or not to show the "Delete User" button.

- Add a short summary of the Ghost User to the bio.

- Have `Uniquify` take a block instead of a Proc/function. This is more
  idiomatic than passing around a function in Ruby.

- Block a user before moving their issues to the ghost user. This avoids a data
  race where an issue is created after the issues are migrated to the ghost user,
  and before the destroy takes place.

- No need to migrate issues (to the ghost user) in a transaction, because
  we're using `update_all`

- Other minor changes

The default (false) is not strictly required, and this lets us avoid a
potentially expensive migration

1. Refactoring and specs in the `Uniquify` class.

2. Don't use the `AdvisoryLocking` class. Similar functionality is
provided (backed by Redis) in the `ExclusiveLease` class.

Rather than using a separate `ghost` state. This lets us have the benefits of
both ghost and blocked users (ghost: true, state: blocked) without having to
rewrite a number of queries to include cases for `state: ghost`.

1. Use an advisory lock to guarantee the absence of concurrency in `User.ghost`,
to prevent data races from creating more than one ghost, or preventing the
creation of ghost users by causing validation errors.

2. Use `update_all` instead of updating issues one-by-one.

1. Create a `Uniquify` class, which generalizes the process of generating unique
   strings, by accepting a function that defines what "uniqueness" means in a
   given context.

2. WIP: Make sure tests for `Namespace` pass, add more if necessary.

3. WIP: Add tests for `Uniquify`

- "Associated" issues are issues the user has created + issues that the
  user is assigned to.

- Issues that a user owns are transferred to a "Ghost User" (just a
  regular user with `state = 'ghost'` that is created when
  `User.ghost` is called).

- Issues that a user is assigned to are moved to the "Unassigned" state.

- Fix a spec failure in `profile_spec` — a spec was asserting that when a user
  is deleted, `User.count` decreases by 1. After this change, deleting a user
  creates (potentially) a ghost user, causing `User.count` not to change. The
  spec has been updated to look for the relevant user in the assertion.

We already skip sending notifications to blocked users. Simply add ghost users
to this list.

Artifact Expiration Defaults

Closes #27762

See merge request !9219

Fixed instructions for adding Secret Variables to a CI/CD pipeline.

See merge request !9480

fixes job dropdown action button error

Closes #27530

See merge request !9182

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9219#note_24032923

* upstream/master: (247 commits)
  Switched CONTRIBUTING.md style guide recommendation for method chaining
  Fix new offenses
  Stylistic tweaks
  Fix OAuth/SAML user blocking behavior
  Revert "Enable Style/DotPosition"
  Revert "Prefer leading style for Style/DotPosition"
  Revert "Enable Style/BarePercentLiterals"
  Manually correct autocorrect
  Move up delegate calls
  Exclude migrations from Style/MutableConstant
  ActiveSupport delegation is preferred over Forwardable
  Update haml_lint to work with newest rubocop
  Add explanations to cops
  Update rubocop and rubocop-rspec and regenerate .rubocop_todo.yml
  Update rubocop and rubocop-rspec and regenerate .rubocop_todo.yml
  Order cops alphabetically
  Don’t exclude some file in lib from rubocop
  Fix new offenses
  Enable Rails/Delegate
  Enable Style/WordArray
  ...

Create master branch first if project is repository-less

Closes #26687

See merge request !9009

Disable unused tags count cache for Projects, Builds and Runners

Closes #27989

See merge request !9467