Introducing Docker Registry replication

- DB migration of board milestone values
- issue finder & spec updates

For customers with thousands of entries in the table, lookups for an
LDAP identity dominated the PostgreSQL statistics because of sequential
scans. We can speed this up by adding a partial index with the
extern_uid and provider.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/59630

These migrations do the following:

- Adds a new `issue_id` column to `versions`. This fixes an n+1 problem
  when loading versions for an issue in GraphQL as AR can now load from
  cache
- Change the unique restraint on versions.sha to be scoped to `issue_id`
  as in order to import version data, we need to allow duplicate `sha`
  values for versions
- Update all versions with an `issue_id`

https://gitlab.com/gitlab-org/gitlab-ee/issues/11090

Adds Job specific variables to facilitate specifying variables when
running manual jobs.

This MR updates the y_label values and the label
values in common_metrics and cluster_metrics to
append the units of measurement to the labels.

Adds event enum column to DesignsVersions join table

See merge request gitlab-org/gitlab-ce!30745

It needs to default to an empty array logically.

`Array.reverse_each` is faster than `Array.reverse.each` because:

* reverse.each creates a new array then loops each element
* reverse_each loops in reverse order (no intermediate array created)