Update /firefox/whatsnew with FxOS messaging. Bug 890114.

- Specified locales get FxOS messaging.

- Old messaging retained for legacy/other use.

Needs a review of Python/Django stuff by @pmclanahan or @jgmize as view handling for /whatsnew and /firstrun has changed significantly.

If direction/foundation is good, I'll write tests against the new view handlers.

Front-end code was previously r+'d by @alexgibson.