Updates to Send to phone feature

These updates should make the send to phone feature a bit more error-proof and user-friendly.

- Restrict feature to US ip addresses only, using the existing Geocoder gem integration (`request.location.try(:country_code) == 'US'`)
- Change phone number mask from Josh Bush's [masked-input-plugin](http://digitalbush.com/projects/masked-input-plugin) to Igor Escobar's [jquery-mask-plugin](http://igorescobar.github.io/jQuery-Mask-Plugin/)
- Require complete US phone number before submission
we don't usually use jquery in blockly apps (it used to be able to be run as a standalone module with node.js and there was no jquery in that mode). however, we do use it sometimes when supplied by the dashboard.

So this is probably safe since this mode is only enabled when the dashboard tells blockly that it should be.