Permissions improvements

This gets rid of explicit thread-locked checks and gets them handled in *permissions.can_...()* methods. It gives us elegant way to allow moderators to edit all posts and post in locked threads. Explicit notifications that thread is locked are removed, however user should not be able to see them since buttons to edit/post are already hidden for locked threads. The only instance i can think of is thread being locked while user is writing a post, but that should not happen often and permission error should be good enough. Banned users are devoid of any permissions. They still can log in though.