Add (minimal) UI for subscriptions

Added `SubscriptionModule` to handle subscriptions views:

 * Inject a (disabled) button into `changeset`, `browser`, and
 `attachment` pages.
 * Handle `GET`, `POST`, and `PUT` requests for subscriptions, accepting
 and returning only JSON content

Added `Subscription.for_request()` to get or create a subscription from
a request.

Added Backbone `Subscription` model and `SubscriptionView` view to
handle UI interaction.

Modified email template to let users know how to unsubscribe.