Add ability to subscribe to text messages to passengers

[Anbranin]

Some passengers would like to receive communication via text versus over the phone, as is done currently. It sure would be nice from an accessibility standpoint to be able to text passengers in an official manner (and make sure they're opted-in). The solution decided upon is that the dispatchers or admins can use email to send texts to passengers when that feature is available.

This means that we need to add two columns to Passenger: A boolean like "subscribed_to_sms" with a default of false, and a carrier_id related to a new model "Carrier". (or CellCarrier, whatever).

A Carrier should have a gateway address and a name. An example is Verizon. The name is "Verizon" and the gateway address is "vztext.com". To text a Verizon number, you'd email [10-digit cell number]@vztext.com.

A page to manage carriers is required here, basically a scaffold, maybe some help text indicating how things work would be nice. An additional nicety would be to construct an email address for each passenger based on their cell number and gateway and put that on their show page. It's certainly possible.

Make sure that a couple example cell carriers are added to the seeds file.