Wordfilter is unused in current gochan codebase,
Remove migrating sql to make it simpler
Make staff reference in wordfilter non-null again
(Making it nullable was a compromise for migrating)
Added is_top_post to posts because you cannot have mutually dependent
entries (original plan was to have thread point to its own top post to
avoid costly constant sort operations).
Indexed posts.is_top_post and threads.is_deleted