Expose DesignType#notes_count in GraphQL

What does this MR do?

• This MR exposes a new #user_notes_count method on Design as DesignType#notes_count in GraphQL. The method uses a new DesignUserNotesCountService which inherits from BaseCountService and caches the count.
• New after_commit callbacks have been added to Note to call #after_note_created and #after_note_destroyed on their noteable, so the noteable can perform tasks particular to them, like cache expiry.
• Refactors into a shared example some spec behaviour tests of a counter caching service

CE backport https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/32037.

Issue #13353 (closed).

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers