Added Locale Service. Minor Controller Refactoring.

Hi there!

I started extracting some "buisiness" logic into a seperate provider/service. My goal, also in preparation for angular 2.0 some day, is to have as much of non binding logic to fit in services. So the migration won't be to much of a pain one day.

I started with this small chunk because it was just easy to extract. So that the monster SyncthingController can shrink in size over time.

Regards,
Dennis
What we get from `/lang` is actually the list of locales requested by the browser (because they are not readily available from Javascript). So this is technically `readRequestedLocales()` or something like that.