[#1163] Users can be deleted

To do this, I've configured the User model to be stateful using vdm.sqlalchemy.
Right now, there're two states: active and deleted. If a user is deleted, he
can't login, and is unauthorized to do anything. She also doesn't appear in the
user's list anymore, but you can still access her profile page, if you know her
username.

If she was logged in when her user was deleted, the next time she goes into
CKAN, she'll be logged off. Unfortunately, there's not a useful message like
"Your user has been deleted." Yet.

There's no way to undelete a user, but it should be simply creating an
action to set her state to active.