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.
Small thing, but it might be a bit easier to understand if we specified units:

```ruby
if @redis.setnx(@key, 20.seconds.from_now.to_i)
  @redis.expire(@key, 20.seconds)
```

The second change is a noop. `20.seconds` returns `20. The first change makes a bit of a change that we'd have to account for below (I'll note it): it implies `Time.zone.now` rather than `Time.now`. In this case, it doesn't matter which one we use as long as we're consistent.