Notes call `#after_note_created` and `#after_note_destroyed` on their
noteable in callbacks, so the noteable can perform tasks particular to
them, like cache expiry.
This is in preparation of the EE-specific class
`DesignManagement::Design` clearing its `user_notes_count` cache when
its note are created or destroyed.
Refactoring Rspec behaviour testing of a counter caching service into a
shared example.
https://gitlab.com/gitlab-org/gitlab-ee/issues/13353
By caching the number of personal SSH keys we reduce the number of
queries necessary on pages such as ProjectsController#show (which can
end up querying this data multiple times).
The cache is refreshed/flushed whenever an SSH key is added, removed, or
when a user is removed.
