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.
Instead of updating `zach` after fetching him from the database, can we have a user fixture where this is already true? Doing the update makes an extra query per test, which we don't need.