Improvements to SMS interactions when using mass SMS and Clickatell

Committing various improvements we made to Mass SMS interactions when using Clickatell as a provider. As this splits the SMS activity from "SMS" into "Inbound SMS" and "SMS delivery" (which I think is a nice improvement rather than using subject), some type of migration script would be needed. Will get Michael McAndrew to comment about the code regarding "$default_country_code='44';".