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: it doesn't matter in this particular case, but it can also be more precise to use ```this.sinon.stub(navigator.mozL10n, 'get').withArgs(title).returns(localizedTitle);```