Feature/digests

@jarsbe finally finished this off today when I got in the mood to do some spec practice.

Relatively simple, adds a settings page for users, adds a flag called `send_digest` to the `User` model, and adds a mailer to send the digests each week. 

At the moment I've got it configured via `whenever` to send the digests every week at 9am on a Friday (we'll need to make sure our server time is in NZDT).

Also adds a new concern called `Tokenable` which is used for token generation as well as finding users by token using `ActiveSupport::MessageVerifier`. At the moment I'm using this for the unsubscribe link in the digest.