We want to run CI with rails 4 for a short-term (until we are sure that
we will ship with rails 5). The problem is that rails 4 can not handle
rails 5 schema.rb properly - specifically `t.index` directive can not
handle multiple indexes on the same column.

Because combination of rails 4 + rails 5 schema will be used
only in CI for a short-term, we can just ignore these incompatibility
failures. This patch adds `rails5` helper for specs.

* add_index replaced with t.index
* intiger(size 8) replaced with bigint

* similar to rails 5 it assures that index length is set for blob
  columns also in rails 4
* it also ignores multiple definitions of indexes for mysql, for some
  tables we define multiple indexes on the same set of columns, but with
  different parameters (opclasses, where), these are not supported by
  mysql adapter so the second definition of index is skipped

CI jobs will be triggered both with rails 4 and 5
to make sure we keep backward compatibility if it turns out
we have to switch back to rails 4.

Rails 4 jobs are not allowed to fail for now, these jobs will be
removed in a follow-up MR next cycle.

* updates Gemfile
* uses Rails 5 unless explicitly disabled

Resolve "Empty label menu if not logged in"

Closes #53816

See merge request gitlab-org/gitlab-ce!23042

Revert "Merge branch 'revert-drop-gcp-clusters-table' into 'master'"

See merge request gitlab-org/gitlab-ce!23020

Avoid returning deployment metrics url to MR widget when the deployment is not successful

Closes #53870

See merge request gitlab-org/gitlab-ce!23010

Track Kubernetes errors using Sentry

Closes #53922

See merge request gitlab-org/gitlab-ce!23026

Update comment in Auto-DevOps.gitlab-ci.yml

See merge request gitlab-org/gitlab-ce!23013

Adds documentation of git trace when git push/pull times out

See merge request gitlab-org/gitlab-ce!23016

Fix a bug where internal email pattern wasn't respected

Closes #52940

See merge request gitlab-org/gitlab-ce!22516

Update events.md.

See merge request gitlab-org/gitlab-ce!23021

Add docs for setting up external Prometheus

Closes #40949 and #37255

See merge request gitlab-org/gitlab-ce!22831

Fix a race condition in the shard population logic

Closes #53972

See merge request gitlab-org/gitlab-ce!23028

Upgrade to Ruby 2.5.3

Closes #41825

See merge request gitlab-org/gitlab-ce!22555

Fixed image commenting on moved files

See merge request gitlab-org/gitlab-ce!23023

Fix broken UI on commit discussions

Closes #53780

See merge request gitlab-org/gitlab-ce!22952

Merge branch '53812-review-apps-check-whether-release-exists-before-attempting-stop_review' into 'master'

Ensure we abort the cleanup if release is empty

Closes #53812

See merge request gitlab-org/gitlab-ce!22992

Border radius fixes for redesigned discussions

Closes #53880 and #53767

See merge request gitlab-org/gitlab-ce!23006

Updates ci header component with gitlab-ui

See merge request gitlab-org/gitlab-ce!23000

Signed-off-by: Rémy Coutable <remy@rymai.me>

Update URL for the grafana instance of sitespeed

See merge request gitlab-org/gitlab-ce!23001

Update docs to put prepend as the last line

Closes #53919

See merge request gitlab-org/gitlab-ce!23017

Add documentation about frontend test categories to frontend guide

See merge request gitlab-org/gitlab-ce!21344

Document new report types

See merge request gitlab-org/gitlab-ce!22828