Post URLs

We now keep track of a post's post number, and a thread's latest post number. On the client, `/t/:slug/:id/:post_number` will link directly to a post. Additionally, instead of keeping track of timestamps to determine whether a person has visited a thread, we keep track of the last post number they read. This way, when linking to a thread that's unread, we can link to the next unread post for that user instead of going right to the top of the thread.

#144 will need to be changed at some point (depending on the order in which these are merged) to take these changes into consideration.
Given when this migration runs, is it true that `visitable_type` is always "DiscussionThread?" You seem to be assuming this in `down` but not in `up` and I want to double check.