Use foreign key to get latest MR diff

Closes #37631

See merge request gitlab-org/gitlab-ce!15126

Fix link text from group context

Closes gitlab-ee#4100

See merge request gitlab-org/gitlab-ce!15565

Allow password authentication to be disabled entirely

Closes #37320

See merge request gitlab-org/gitlab-ce!15223

Remove cluster_id from 20171013104327_migrate_gcp_clusters_to_new_clusters_architectures

See merge request gitlab-org/gitlab-ce!15566

Cross link default branch docs to issue closing pattern docs

Closes #40449

See merge request gitlab-org/gitlab-ce!15546

Fix the redirect location wording

Closes gitlab-com/gitlab-docs#142

See merge request gitlab-org/gitlab-ce!15572

Compared to the merge_request_diff association:

1. It's simpler to query. The query uses a foreign key to the
   merge_request_diffs table, so no ordering is necessary.
2. It's faster for preloading. The merge_request_diff association has to load
   every diff for the MRs in the set, then discard all but the most recent for
   each. This association means that Rails can just query for N diffs from N
   MRs.
3. It's more complicated to update. This is a bidirectional foreign key, so we
   need to update two tables when adding a diff record. This also means we need
   to handle this as a special case when importing a GitLab project.

There is some juggling with this association in the merge request model:

* `MergeRequest#latest_merge_request_diff` is _always_ the latest diff.
* `MergeRequest#merge_request_diff` reuses
  `MergeRequest#latest_merge_request_diff` unless:
    * Arguments are passed. These are typically to force-reload the association.
    * It doesn't exist. That means we might be trying to implicitly create a
      diff. This only seems to happen in specs.
    * The association is already loaded. This is important for the reasons
      explained in the comment, which I'll reiterate here: if we a) load a
      non-latest diff, then b) get its `merge_request`, then c) get that MR's
      `merge_request_diff`, we should get the diff we loaded in c), even though
      that's not the latest diff.

Basically, `MergeRequest#merge_request_diff` is the latest diff in most cases,
but not quite all.

Updated all Vue libraries

See merge request gitlab-org/gitlab-ce!15554

Improve environments performance

Closes #32098

See merge request gitlab-org/gitlab-ce!15484

Closes https://gitlab.com/gitlab-com/gitlab-docs/issues/142

Resolve "EE Compatibility Check Job Fails Unexpectedly"

Closes #40463

See merge request gitlab-org/gitlab-ce!15555

Fix encoding bugs in Gitlab::Git::User

Closes #40330

See merge request gitlab-org/gitlab-ce!15539

Merge branch 'fix/sm/37991-avoid-deactivation-when-pipeline-schedules-execute-a-commit-includes-ci-skip' into 'master'

Avoid deactivation when pipeline schedules execute a branch includes `[ci skip]` comment

Closes #37991

See merge request gitlab-org/gitlab-ce!15405

Add environment_scope to cluster table

See merge request gitlab-org/gitlab-ce!15419

Resolve "Hide `section_start` and `section_end` in pipeline emails"

Closes #39653

See merge request gitlab-org/gitlab-ce!15534

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

Resolve "Upgrade FE Babel Libraries"

Closes #40445

See merge request gitlab-org/gitlab-ce!15544

Change star, filter and settings icons to the their sprite based replacements

Closes #33757

See merge request gitlab-org/gitlab-ce!15412

[ci skip]

Speed up Unicorn specs by using a dummy Rack application instead of GitLab

Closes #32620

See merge request gitlab-org/gitlab-ce!15548

Add edit button to mobile file view

Closes #28377

See merge request gitlab-org/gitlab-ce!15199

Fix missing MR widget status icons

Closes #40283

See merge request gitlab-org/gitlab-ce!15533

Resolve "Upgrade axios from 0.16.2 to 0.17.1"

Closes #40444

See merge request gitlab-org/gitlab-ce!15543

Temporarily force disable prometheus metrics

See merge request gitlab-org/gitlab-ce!15549

Until https://gitlab.com/gitlab-org/prometheus-client-mmap/merge_requests/11
is ready, Prometheus metrics will not work and cause issues such as #40457.

Replace vue-resource for axios in the Prometheus Dashboard

See merge request gitlab-org/gitlab-ce!15547