Remove SMS from Scout

This removes SMS and Twilio integration from Scout. 

This also means removing the special remote endpoint that allowed [Call on Congress](http://calloncongress.sunlightfoundation.com/) users to receive whitelabeled SMS alerts via Scout. Call on Congress has, separately, been updated to remove the option for users to sign up for SMS alerts. 

Users are now required, unequivocally, to have a unique, present email address. I've removed all references to the `user_contact` helper, and user IDs are always email. I could go further and remove some  additional fields (like `mechanism` on `Receipt` and `Delivery`) but I haven't bothered doing so at this time.

On the user Settings page, the SMS configuration area has been replaced with a mini-guide to using Scout's RSS feeds to set up SMS alerts in [IFTTT](https://ifttt.com), via a [shared recipe](https://ifttt.com/recipes/161435-get-my-scout-alert-via-sms) I created on behalf of Sunlight that eases the process. This configuration area appears as the below:

![screenshot from 2014-04-11 14 11 40](https://cloud.githubusercontent.com/assets/4592/2683071/df428904-c1a4-11e3-97cb-841a8ff09049.png)

For easier portability to non-Sunlight contexts, that area could be made more configurable in the future -- the only necessity would be switching the IFTTT recipe being embedded.