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.

This closes #105.
Unfortunately our Mail objects don't actually end up in `Actionmailer::Base.deliveries` (except for mentions) because we deliver through Mailgun's API.