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
Yeah, the report/feedback process requires dashboard integration, and jquery has been required by our dashboard integration since at least March (commit [2a4e06](https://github.com/code-dot-org/dashboard/commit/2a4e06a7947cec48707fe5c4e8d322a453abf43b)). I don't think this will be a problem unless we imagine we'll need to remove our existing jquery dependency from Dashboard down the road.