Prepares us for upgrade to Rails 5.2

Fix pipelines for merge requests does not show pipeline page when source branch   is removed

Closes #60432

See merge request gitlab-org/gitlab-ce!27803

Source branch can be removed after the merge and
we have to make sure to avoid rendering links if it's the case.

Docs: Add intro and reorg CI index

Closes #58323

See merge request gitlab-org/gitlab-ce!27620

Plus: add note for .com users

Merge branch '60476-actionview-missingtemplate-missing-template-projects-issues-_merge_requests-with-locale-en-formats-html-varian' into 'master'

Delete leftover code for referenced_merge_requests endpoint

Closes #60476

See merge request gitlab-org/gitlab-ce!27335

Fixed boards card dragging styling

Closes #61050

See merge request gitlab-org/gitlab-ce!27802

Fix base domain help text update

Closes #61036

See merge request gitlab-org/gitlab-ce!27746

Internationalisation of javascript/m* directories

See merge request gitlab-org/gitlab-ce!27723

This is one of many MRs opened in order to improve the overall
internationalisation of the GitLab codebase.

i18n documentation
https://docs.gitlab.com/ee/development/i18n/externalization.html

Internationalisation of vue_shared directory

See merge request gitlab-org/gitlab-ce!27751

This is one of many MRs opened in order to improve the overall
internationalisation of the GitLab codebase.

This commit only targets the Vanilla JS files.

i18n documentation
https://docs.gitlab.com/ee/development/i18n/externalization.html

Update dependency @gitlab/ui to ^3.7.0

See merge request gitlab-org/gitlab-ce!27832

Resolve "Project is being serialized when enqueueing to sidekiq"

Closes #59265

See merge request gitlab-org/gitlab-ce!27825

Prevent concurrent execution of PipelineScheduleWorker

Closes gitlab-com/gl-infra/production#805

See merge request gitlab-org/gitlab-ce!27781

Change comments to point to the new path of sidekiq-cluster

See merge request gitlab-org/gitlab-ce!27707

Upgrade letter_opener_web to support Rails 5.1

See merge request gitlab-org/gitlab-ce!27829

Before this commit using `/rails/letter_opener` in `development`
environment failed with:

    undefined method `before_filter'

See https://github.com/fgrehm/letter_opener_web/issues/68

This commit upgrades `letter_opener_web` to 1.3.4 so Rails 5.1 is
supported.

Add full backtrace for RSpec output

See merge request gitlab-org/gitlab-ce!27686

Upgrade Gitaly to v1.36.0

See merge request gitlab-org/gitlab-ce!27831

Make `CI_COMMIT_REF_NAME` and `SLUG` variable idempotent

Closes #60822

See merge request gitlab-org/gitlab-ce!27663

Move test method to the bottom

See merge request gitlab-org/gitlab-ce!27816

Internationalisation of cycle_analytics directory

See merge request gitlab-org/gitlab-ce!27674

This is one of many MRs opened in order to improve the overall
internationalisation of the GitLab codebase.

i18n documentation
https://docs.gitlab.com/ee/development/i18n/externalization.html

Format extra help page text like wiki

Closes #55948

See merge request gitlab-org/gitlab-ce!26782

CE Backport: Prefer usage of helper `expanded_by_default?`

See merge request gitlab-org/gitlab-ce!27729

Stop using `Rails.env.test?` in views.

CE: Fixes EE differences for app/views/projects/new.html.haml

See merge request gitlab-org/gitlab-ce!27617

Use keyword args for databuilder push

See merge request gitlab-org/gitlab-ce!24088

`Todos::Destroy::EntityLeaveService#project_ids` was returning
ActiveRecord objects with IDs, not simply IDs. That means we were
serialising more than we needed to in Sidekiq.

We can simply rename this method to `#projects` as this class doesn't
use any of the superclass methods that would use `#project_ids`.

Internationalisation of groups directory

See merge request gitlab-org/gitlab-ce!27679

This is one of many MRs opened in order to improve the overall
internationalisation of the GitLab codebase.

i18n documentation
https://docs.gitlab.com/ee/development/i18n/externalization.html

Attempt to debug https://gitlab.com/gitlab-org/gitlab-ce/issues/60953

Fix slow performance with compiling HAML templates

Closes gitlab-ee#11198

See merge request gitlab-org/gitlab-ce!27782

Disallow read user scope to read project events

See merge request gitlab/gitlabhq!3067

In Rails 5, including `ActionView::Context` can have a significant and
hidden performance penalty because this module also includes
`ActionView::CompiledTemplates`. This means that any module that
includes ActionView::Context becomes a descendant of
`CompiledTemplates`.

When a partial is rendered for the first time, it runs
`ActionView::CompiledTemplates#module_eval`, which will evaluate a
string that defines a new method for that partial. For example, the
source of partial might be this string:

```
def _app_views_project_show_html_haml___12345(local_assigns, output)
  "hello world"
end
```

When this string is evaluated, the Ruby interpreter will define the
method and clear the global method cache for all descendants of
`ActionView::CompiledTemplates`. Previous to this change, we
inadvertently made a number of modules fall into this category:

* GroupChildEntity
* NoteUserEntity
* Notify
* MergeRequestUserEntity
* AnalyticsCommitEntity
* CommitEntity
* UserEntity
* Kaminari::Helpers::Paginator
* CurrentUserEntity
* ActionView::Base
* ActionDispatch::DebugExceptions::DebugView
* MarkupHelper
* MergeRequestPresenter

After this change:

* Kaminari::Helpers::Paginator
* ActionView::Base
* ActionDispatch::DebugExceptions::DebugView

Each time a partial is rendered for the first time, all methods for
those modules will have to be redefined. This can exact a significant
performance penalty.

How bad is this penalty? Using the following benchmark script, we can
use DTrace to sample the Ruby interpreter:

```
Benchmark.bm do |x|
  x.report do
    1000.times do
      ActionView::CompiledTemplates.module_eval("def testme\nend")
    end
  end
end
```

This revealed a 11x jump in the time spent in `core#define_method`
alone.

Rails 6 fixes this behavior by moving the `include CompiledTemplates`
into ActionView::Base so that including `ActionView::Context` doesn't
quietly affect other modules in this way.

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/11198