Better handle lack of permissions when quoting, editing, or posting.

* Quotes and edits fail with a more specific "you're not allowed" error message
  when appropriate. Unfortunately there's no difference in markup to indicate
  when a quote/edit would be allowed (the buttons are hidden using CSS on the
  Forums proper), so we're stuck with this for now.
* The "post thread" button only enables when the forum will allow a new thread.