This commit does two things:
1. Synchronizes `Gemfile.rails5.lock` with the latest changes in `Gemfile`
2. Adds a monkey patch for active record to avoid type casting in uniqueness
validator.

In Rails 5.0 there was introduced a bug [1] that causes to problems like
this:

```
1) API::Users POST /user/:id/gpg_keys/:key_id/revoke when authenticated revokes existing key
Failure/Error: let(:gpg_key) { create(:gpg_key, user: user) }
TypeError:
  can't cast Hash
```

This bug was fixed in Rails 5.1 [2].

[1]: https://github.com/rails/rails/pull/23523/commits/811a4fa8eb6ceea841e61e8ac05747ffb69595ae
[2]: https://github.com/rails/rails/pull/24745/commits/aa062318c451512035c10898a1af95943b1a3803

Resolve "View labels when editing labels in sidebar and board sidebar"

Closes #34262

See merge request gitlab-org/gitlab-ce!18380

Bump CI to use Chrome V65 and ChromeDriver 2.36

See merge request gitlab-org/gitlab-ce!18459

Revert the addition of goldiloader

See merge request gitlab-org/gitlab-ce!18458

Resolve "Avoid possible infinite loops in Container Scanning"

See merge request gitlab-org/gitlab-ce!18467

Fix code climate example

See merge request gitlab-org/gitlab-ce!18267

Fix issues on groups group trees

Closes #42407

See merge request gitlab-org/gitlab-ce!18390

Fix Custom hooks are not triggered by UI wiki edit

Closes #42354

See merge request gitlab-org/gitlab-ce!18251

Fix DAST example in documentation

See merge request gitlab-org/gitlab-ce!18456

This reverts commit 2c3c5b35.

Revisit this after https://gitlab.com/gitlab-org/gitlab-ce/issues/45441

This reverts the addition of the "goldiloader" Gem and all use of it.
While this Gem is very promising it's causing a variety of problems on
GitLab.com due to it eager-loading too much data in places where we
don't expect/can handle this. At least for the time being this means we
have to go back to manually fixing N+1 query problems, but at least
those should not cause a negative impact on availability.

set proper signature marker in mails

Closes #43617

See merge request gitlab-org/gitlab-ce!17979

Add more detail to cleanup steps for background migrations

See merge request gitlab-org/gitlab-ce!18448

Fix regression around new SHA widget in "New Merge Request" Screen

Closes #45446

See merge request gitlab-org/gitlab-ce!18422

Improve web IDE commit input

Closes #44832

See merge request gitlab-org/gitlab-ce!18389

Ensure that cached_markdown_version is handled correctly for CommonMark

See merge request gitlab-org/gitlab-ce!18431

[Rails5] Add `safe_params` helper

See merge request gitlab-org/gitlab-ce!18241

1. We can't just steal from the queue, in case there was a problem with Sidekiq.
2. We need to consider import / export.

Signed-off-by: Enrico Scholz <enrico.scholz@sigma-chemnitz.de>

use rfc 3676 mail signature delimiters ("-- ") instead of the previous
"---" one.

closes #43617
Signed-off-by: Enrico Scholz <enrico.scholz@ensc.de>

This reverts merge request !18399

Fix link in Security Developer Workflow.md

See merge request gitlab-org/gitlab-ce!18445

Email notification on issue due date

Closes #27500

See merge request gitlab-org/gitlab-ce!17985

Resolve "Make a Rubocop that forbids returning from a block"

Closes #42889

See merge request gitlab-org/gitlab-ce!18000

Resolve "Concurrent requests to api/v4/jobs/request cause DB locks on ci_builds_metadata table"

Closes #45456

See merge request gitlab-org/gitlab-ce!18428

Resolve "Using goldiloader results in Ci::RegisterJobService#execute spending a lot of time eager loading a lot of associations"

See merge request gitlab-org/gitlab-ce!18439

Resolve "Time estimate system notes can be confusing"

Closes #40402

See merge request gitlab-org/gitlab-ce!18326

Docs assign labels to subgroups

See merge request gitlab-org/gitlab-ce!18438

Fix Markdown typo in Web IDE docs

See merge request gitlab-org/gitlab-ce!18443

Fix `Trace::HttpIO` can not render multi-byte chars

Closes #44981

See merge request gitlab-org/gitlab-ce!18417

Improve jobs queuing time metric (fixed version)

Closes #45205

See merge request gitlab-org/gitlab-ce!18283