Prior to this commit, the `index_statuses` table was allowed to go stale after
initial indexing. This made it impossible to provide status details, and also
made treating the initial index as a 'backfill' job harder.

Now, if a repository is indexed via `git push`, the initial indexing will
recognise that it has been indexed, and not re-index it.

We introduce an asynchronous initial indexer and an index status rake task at
the same time.

This commit introduces a `gitlab:elastic:index_repositories_async' rake task,
which can be used to enqueue a set of indexing jobs into Sidekiq. These will
be scheduled in the usual manner, in the `sidekiq` queue.

In theory, this allows Sidekiq's concurrency and distributed execution features
to be used to spread and control the load of indexing repositories.

CE upstream

Closes #395

See merge request !1161

Fix "NameError: uninitialized constant Rack::Proxy" in production environment

See merge request !1168

UX Guide: Button placement in groups

See merge request !8972

Integrate webpack for frontend asset compilation

Closes #14634

See merge request !7288

Port "Integrate webpack for frontend asset compilation" to EE

See merge request !1138

* master: (54 commits)
  Move project services to new location under Integrations
  Move webhooks to new a location under Integrations
  Fix documentation link in doc/user/project/pages/index.md
  Fixed merge request widget tests
  Small documentation changes and fixes for Geo
  Remove ‘EE-specific’ comments related to Pages feature
  Update specs with new method signatures
  rm_tag now takes a user argument
  Use GitOperationService instead
  Merge remote-tracking branch 'ce/fix-git-hooks-when-creating-file' into master-ce
  Mentioned pages to CE port in project/pages/index.md
  Fixed merge request environment not displaying
  Update guidance on handling shared secrets in HA setup
  Pull EE specific Gitlab::Auth code in to its own module
  Use `add_$role` helper in snippets specs
  Check public snippets for spam
  Keep snippet visibility on error
  Add info on using both push/pull repository mirroring
  Properly read approvalPending from dataset.
  Fix data attr spacing.
  ...

Document that the retro and kickoff notes are public

See merge request !8788

* master:
  Change window size before visiting page, to get correct scroll position
  Fix slash commands spec error
  Move project services to new location under Integrations
  Move webhooks to new a location under Integrations
  Fixed eslint test failure
  Fixed redirection from http://someproject.git to http://someproject
  Add traits for the different Event types to the Event factory
  prevent nonewline type diff lines from containing unfolding link
  ensure linenumber data attribute is correct for the last line in a diff chunk
  ensure last line in diff block is contained within a table row element
  ensure click event is bound only once

Resolve "Line numbers are off in dynamically loaded diff chunks"

Closes #26300

See merge request !8428

Change window size before visiting page, to get correct scroll position

See merge request !8963

Fix slash commands spec error

See merge request !8968

Redirect http://someproject.git to http://someproject

Closes #26275

See merge request !8951

Refactor integrations page documentation

See merge request !8966