Lots of sanity-related work happening.

* The article form checks if the user the form says is making
   changes actually exists (though that might be overkill
   considering that flask-login is going to tell us who is
   logged in, not the user)
* auth.User is using flask_login.UserMixin now, still not sure
   if I want to use `id` or `uname` or both somehow.
* Cleaned up navigation, not too different from before the
   changes, but a lot nicer.
* Templates use a user object now, rather than a session one.
* That user object should probably be sanitized to some
   degree; though there isn't really any sensitive info stored
   there. (pw is hashed, &c.)