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.
Didn't know you could do `where(subscribable: @thread)` for polymorphic associations. Assuming this works, it's pretty cool