Add docs explaining why you get signed out with "Remember me" checked

See merge request gitlab-org/gitlab-ce!15756

See https://gitlab.slack.com/archives/C101F3796/p1512486640000434

Check NFS mounts in a separate process

Closes #39847

See merge request gitlab-org/gitlab-ce!15426

Moving the check out of the general requests, makes sure we don't have
any slowdown in the regular requests.

To keep the process performing this checks small, the check is still
performed inside a unicorn. But that is called from a process running
on the same server.

Because the checks are now done outside normal request, we can have a
simpler failure strategy:

The check is now performed in the background every
`circuitbreaker_check_interval`. Failures are logged in redis. The
failures are reset when the check succeeds. Per check we will try
`circuitbreaker_access_retries` times within
`circuitbreaker_storage_timeout` seconds.

When the number of failures exceeds
`circuitbreaker_failure_count_threshold`, we will block access to the
storage.

After `failure_reset_time` of no checks, we will clear the stored
failures. This could happen when the process that performs the checks
is not running.

Fix new personal access token showing up in a flash message

Closes #40754

See merge request gitlab-org/gitlab-ce!15707

Clean test path between merge request handler tests

Closes #40900

See merge request gitlab-org/gitlab-ce!15792

Migrate Git::Repository#fsck to Gitaly

Closes gitaly#795

See merge request gitlab-org/gitlab-ce!15731

Use relative _path helper URLs in the GitLab UI

Closes #40825

See merge request gitlab-org/gitlab-ce!15738

Replace absolute URLs on related branches/MRs with relative url to avoid hostname

Closes #40555

See merge request gitlab-org/gitlab-ce!15735

Fix loading branches list on cherry pick modal after merge.

Closes #33894

See merge request gitlab-org/gitlab-ce!15306

Add documentation about using US-English

Closes #40675

See merge request gitlab-org/gitlab-ce!15656

Fix transient error in pikadayToString

Closes #40858

See merge request gitlab-org/gitlab-ce!15754

fix #39233 - 500 in merge request

Closes #39233

See merge request gitlab-org/gitlab-ce!15774

Dependency validator

Closes #34834

See merge request gitlab-org/gitlab-ce!14009

create_merge_request_handler_spec needs a repository for some tests but
this repository lasts on disk by default which causes failures of other
tests. TestEnv.clean_test_path is used to get rid of the repository
after each test.

Closes #40900

- handle unchanged empty lines in inline diff