Group texting module

This is https://github.com/hobinjk/ESP-Website/tree/group-texting-module with some modifications. Some more modifications may be desired, for example to have portions of the target recipients to be texted at a time through AJAX instead of all at once (which probably takes a long time?).

I think James didn't merge this earlier because it doesn't support contacting a specific class. But probably that should be separate modification, to UserSearchController?