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.
A comment of mine got deleted when you made a change elsewhere. Here's what I wrote:

This is probably not how we want to be testing mailers.

Action Mailer already doesn't deliver mail in test. Instead, all delivered mail is put into the `ActionMailer::Base.deliveries` array, which is reset after every test.

If this were the only issue, we'd be all set. The problem is that our delayed job worker isn't running in our test environment, so our Action Mailer code never gets called. We should disable our delayed job worker in our test environment.

Create a file called `config/initializers/delayed_job.rb` and put this in it:

```ruby
Delayed::Worker.delay_jobs = !Rails.env.test?
```

If we're in the test environment, all of our delayed jobs will just be run immediately, rather than put into the database. Once you have that, you can then make assertions about the `Mail` objects in `ActionMailer::Base.deliveries`