In order to avoid confusion between Gitlab::Importers module and other
like named modules, this renamed it to Gitlab::DatabaseImporters.

Remove CommonMetricsImporter patch and change all references to
CommonMetrics::Importer. Move specs into their appropriate folders. Also
cleans up some common_metric importer namespacing.

Backport of https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14328

See merge request gitlab-org/gitlab-ce!30003

Remove unused Gitaly feature flags

See merge request gitlab-org/gitlab-ce!30302

When GitLab was migrating to Gitaly, feature flags were used. These are
now out of use, and essentially a no-op. But they do make the output of
chatops ugly and the feature table is selected in full by the
application.

This MR introduces tracking of the `rebase_jid` for merge requests. As
with `merge_ongoing?`, `rebase_in_progress?` will now return true if a
rebase is proceeding in sidekiq.

After one release, we should remove the Gitaly-based lookup of rebases.
It is much better to track this kind of thing via the database.

Add released_at field to releases API
Add released_at column to releases table
Return releases to the API sorted by released_at

This new attribute is optional and used when set instead of the default
format `gitlab+deploy-token-#{id}`.

Empty usernames will be saved as null in the database.

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/50228.