Pipeline must be present if "Pipeline must succeed" is set

See merge request gitlab-org/gitlab-ce!29926

Update GitLab Runner Helm Chart to 0.6.0/12.0.0

See merge request gitlab-org/gitlab-ce!29982

When a user sets only_allow_merge_if_pipeline_succeeds, also named
as "Pipeline must succeed" project setting, we require the pipeline
to be present.

This solves race condition issues especially with external CI
providers when a build is triggered externally but no pipelines are
created in GitLab yet.

Document that a head pipeline is expected when using "Pipeline
must succeed" setting. Also explain limitations with the use of
only/except where there may not be any jobs created and the merge
request will not be allowed to be merged.

Changes migration and all other places the attribute is used

Adds an instance setting to limit display of time tracking
values to hours only

Refactor Deployment#cluster to #deployment_platform_cluster

See merge request gitlab-org/gitlab-ce!29961

Add index for pages domain ssl auto renewal
Add PagesDomain.needs_ssl_renewal scope
Add cron worker for ssl renewal
Add worker for ssl renewal
Add pages ssl renewal worker queues settings

Since this scope is used in both the controller and the API, we rename
it to make it clear that it's used for both.