This changes the BackgroundMigration worker so it checks for the health
of the DB before performing a background migration. This in turn allows
us to reduce the minimum interval, without having to worry about blowing
things up if we schedule too many migrations.

In this setup, the BackgroundMigration worker will reschedule jobs as
long as the database is considered to be in an unhealthy state. Once the
database has recovered, the migration can be performed.

To determine if the database is in a healthy state, we look at the
replication lag of any replication slots defined on the primary. If the
lag is deemed to great (100 MB by default) for too many slots, the
migration is rescheduled for a later point in time.

The health checking code is hidden behind a feature flag, allowing us to
disable it if necessary.

correct error message on helm tiller installation timeout

See merge request gitlab-org/gitlab-ce!20683

Add `runner_unsupported` CI failure

See merge request gitlab-org/gitlab-ce!20664

Create update guide from 11.1 to 11.2

See merge request gitlab-org/gitlab-ce!20945

update installation docs branches for source installations

See merge request gitlab-org/gitlab-ce!20944

Support multiple scopes when authing container registry scopes

Closes #48968

See merge request gitlab-org/gitlab-ce!20617

Add support for ssh certificates (internal API)

Closes #34572

See merge request gitlab-org/gitlab-ce!19911

Update test-and-deploy-python-application-to-heroku.md

See merge request gitlab-org/gitlab-ce!20921

Rails5: fix flaky spec

See merge request gitlab-org/gitlab-ce!20953

Resolve "Allow issue's Internal ID (`iid`) to be set when creating via the API"

Closes #1756

See merge request gitlab-org/gitlab-ce!20626

Fix a few disrepencies between CE and EE in the lines ordering

See merge request gitlab-org/gitlab-ce!20937

Resource event model

Closes #47993

See merge request gitlab-org/gitlab-ce!20606

[CE Port]: Implement instance level project templates

See merge request gitlab-org/gitlab-ce!20761

Show one digit after dot in commit_per_day value in charts page.

Closes #26512

See merge request gitlab-org/gitlab-ce!20896

Search for label on project labels page by title or description

See merge request gitlab-org/gitlab-ce!20749

Why and how to enable this is covered in the docs being changed
here. This requires gitlab-org/gitlab-shell@2e8b670 ("Add support for
SSH certificate authentication", 2018-06-14) which has been merged in
and tagged as 8.0.0, so GITLAB_SHELL_VERSION needs to be bumped.

Merging this closes gitlab-org/gitlab-ce#34572 see
gitlab-org/gitlab-shell!207 for the gitlab-shell MR.

Implementation notes:

 - The APIs being changed here are all internal, and their sole
   consumer is gitlab-shell.

 - Most of the changed code is a MR to gitlab-shell, see the
   gitlab-org/gitlab-shell!207 MR. That change covers why only some of
   the internal methods get a new "username" parameter, and why some
   others only get a "user_id".

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

Add local project uploads cleanup task

See merge request gitlab-org/gitlab-ce!20863

Enable no-console ESLint rule for tests

See merge request gitlab-org/gitlab-ce!20812

Fix LFS uploads not working with git-lfs 2.5.0

Closes #49752

See merge request gitlab-org/gitlab-ce!20923