Add support for user banning

[akprasad]

A user with admin permissions should be able to ban a user through the website. A banned user will not be able to make posts or proofing changes.

[AkshayPall]

I think we want to separate deleting users from posts/revisions/etc as there may be useful revisions/posts they posted that we want to retain. In this way, we need to support the idea of a "Deleted" or "Removed" User (zombie delete) to anonymize submissions similar to Reddit.

Off the top of my head, these are the things needed to change

• add a status field to the user table (ACTIVE=0, DELETED=1, BANNED=2)
• update everywhere 'user' name is displayed to check for status
• prevent logins if inactive user
• update all 'post' queries to check if the user is active (in the race condition where a currently active user is banned - we want to prevent them from taking further actions right away)