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.
Mary was our tie-breaker and sided with me, citing the following rule as a precedent:

Foo bar (baz).
(Foo bar baz.)

That is, if the delimiters surround the entire sentence, the period goes inside them. If the delimiters surround only a fragment, the period goes outside them.