This fixes https://gitlab.com/gitlab-org/gitlab/issues/37766 which is
caused by the fact that we leave the stale permissions data in the index
after a group is moved to another group.

These are not required because MySQL is not
supported anymore

- Externalize strings in milestones_helper
- Externalize strings in app/services
- Update PO file

- we now use the hierarchy class also for epics
- also rename supports_nested_groups? into supports_nested_objects?
  - move it to a concern

This whitelists all existing offenses for the various CodeReuse cops, of
which most are triggered by the CodeReuse/ActiveRecord cop.

For directories application_settings --> labels.

Partially addresses #47424.