Add caching to DraftNote

In https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4213 we did not add caching as it ended up being complicated to filter away draft notes quick actions on the HTML without getting rid of them on note.

Maybe a different approach (separate the quick actions into another persisted attribute?) would be simpler

cc/ @jramsay this might not be needed (draft notes are supposed to be very short lived after all and only the creator can see them) but it's good to evaluate