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