Autosubscribe to new threads and unsubscribe links in email

There's now a new user setting: "Subscribe me to new threads created in subforums I'm subscribed to." This will default to true. Since this means more email volume by default, this pull request also includes support for unsubscribing from threads or subforums using a link sent in the email.
I think you also need to check that the subscription is actually subscribed, right? They could have unsubscribed from a subfourm and still have a subscription.

If you want, you can combine this line and line 33 like so:

```ruby
where(users: {subscribe_new_thread_in_subscribed_subforum: true}, subscriptions: {subscribable_type: "Subforum", subscribable_id: @thread.subforum.id, subscribed: true})
```

Only do it if you think it looks clearner.