Handle deleted users

First commit is removing user from the groups he his following when deleting his account.
Second commit is handling timeline's bug for users who deleted their accounts before the fix.

Second commit might not be necessary, depending on whether or not one prefer fixing issue in Cassandra with a migration script or so.

When the user being deleted is admin of any group, these groups might be left without any admin, and I don't know what to do in those cases :(