Bug 870177 - [SMS / MMS] behavior when character limit of SMS is reached V1.1 r=julienw r=djf

Adds Compose.type - 'sms' or 'mms'
Adds a 'type' event to Compose that is cancelable and called when the type switches
Adds a banner message for when the type switches
Changes logic for dealing with SMS segments to just switch Compose.type to mms when you go over and forces type to stay mms while you stay over the segment limit.
Adds a temporary bit of text saying MMS (Visual Design for an MMS corner banner will happen later)
Bug 840035 - [MMS][User Story] Operator-defined limit prompt r=julienw r=djf

Compose.lock to avoid new characters being entered
Adding over limit prompts back in.