This makes the outcome if a user didn't have an avatar due to a past
email change reasonable; the user will just be bumped back to
gravatar, fixing their invalid state.
This commit introduces a migration for moving avatars from email based
to user id based storage.
This is in responce to change in behaviour of user_avatar_path to
return path comprising of realm id and a hash based on user id. Also
we fix test_helpers accordingly.
Fixes#3776.