[JRO] mv filter from user pref to a board setting

* Remove filter from users' messageboard preferences
* Add the filter field to the messageboard model
* Remove filter related code
* Fix the last user for topic. Was last_topic.user -- should be last_topic.last_user
* Delete NullMessageboardPreference
* Add filter to posts being created
* Fix `find`s that weren't using friendly_id's scoping
* Ensure filters are only either markdown or bbcode
* Get rid of all textile-related code and dependencies
* Convert posts using textile to markdown