Bug 1051793 - [CMAS] Plug in real CMAS input to the CMAS application

- Add cellbroadcast system message handler
- Removed unused styling and testing UI
- Add localization properties
nit: what about ```require('/shared/test/unit/mocks/mock_navigator_moz_set_message_handler.js');```? But to be honest I'm not sure if it's better or not, personally I'd rather migrate that mock to sinon :)