Add firehose subscription to Trac Notifications page

[ericlewis]

Currently, the trac "firehose" requires subscription to a ​separate mailing list. We should integrate this into the ​user notification settings page. Although the fact that it isn't there already leads me to think there's a technical hurtle here.

[Otto42]

The mailing list is somewhat more efficient than the notifications code. It's not that it couldn't be done, but it would kind of be reinventing the wheel, and also making it more pentagonal. :)

[nacin]

If you're going to subscribe to the entire thing, you're better off subscribing to the mailing list, as it'll allow you to better manage things within your inbox. We could have added it, but as @Otto42 says, it'd be less efficient.