Subscriptions

This pull request introduces email subscriptions and refactors our existing email notifications to avoid sending multiple emails to one person about the same thing.

**Summary:** An email will be sent to a user if a new post is made in a thread to which they're subscribed or if a new thread is created in a subforum to which they're subscribed. A user can manually subscribe to individual threads or subforums. Additionally, users are auto-subscribed to threads they create or are `@mentioned` in. (This behavior is governed by user settings.)
If you include the subforum name, I'd do it second. That way if the subject gets truncated, we still see the thread title, which is more important:

```ruby
%{New thread "#{thread.title}" in #{thread.subforum.name}}
```

I don't know about the quotes. We should have a grammar discussion and decide :)