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 that `last_post_number_read` can't be nil, I think it's enough for this to read

```ruby
def next_unread_post_number
  if unread?
    last_post_number_read + 1
  end
end
```