Add endpoint to delete/uninstall a cluster application

Add specs

Only prometheus can be uninstalled atm, the rest will be dealt with
later.

Presumption is that new application types will have uninstallation
implmemented at the same time.

Rescue and put into :uninstall_errored if something goes wrong while
destroying, which can happen. I think it is safe to expose the full
error message from the destroy error.

Remove the :uninstalled state as no longer used.

+ to monitor progress of uninstallation pod

Added :uninstalled state as wasn't sure if we should be destroying the
cluster apps

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`.