Automatically update MR merge-ref along merge status

See merge request gitlab-org/gitlab-ce!29569

This eliminates many potential duplicate FindCommit RPCs for the same
ref, which often occurs in the RelativeLinkFilter#current_commit call.
On the GitLab 12.0 release post, for example, this would save close to
400 RPC calls.

Add ability to reorder issues

See merge request gitlab-org/gitlab-ce!29012

Adds enum certificate_source to pages_domains table
with default manually_uploaded

Mark certificates as 'gitlab_provided'
if the were obtained through Let's Encrypt

Mark certificates as 'user_provided' if they were uploaded through
controller or api

Only show private key in domain edit form if it is 'user_provided'

Only show LetsEncrypt option if is enabled by application settings
(and feature flag)

Refactor and fix some specs to match new logic

Don't show Let's Encrypt certificates as well

to support manual sorting on the frontend

This couples the code that transitions the `MergeRequest#merge_status`
and refs/merge-requests/:iid/merge ref update.

In general, instead of directly telling `MergeToRefService` to update
the merge ref, we should rely on `MergeabilityCheckService` to keep
both the merge status and merge ref synced. Now, if the merge_status is
`can_be_merged` it means the merge-ref is also updated to the latest.

We've also updated the logic to be more systematic and less user-based.

This commits adds support for metrics dashboards
for embedding. If the flag 'embedded' is provided
to the environments/id/metrics_dashboard endpoint,
the response will be suitable for embedding in
issues or other content.

This is a precursor for support for embedding
metrics in GFM.

This adds missing preloads, and introduces
additional n+1 matcher to look for
duplicates.

This commit adds
 - feature specs
  - to test the ability of a user with "developer" permission
    to delete tags in repositories.
 - documentation

This brings the API permissions in line with the UI permissions