Store a reference to the Angular injector

While trying to spike the routing code, I found that every time you call
angular.injector(['App']), it runs all the App’s config functions, which
includes routing setup. It doesn’t seem right.
make: jshint before building html
Consider main() tests integration tests

Everything started with some tests failing when I was offline. Then I found
that external-services’ tests want to connect to the real thing. So I tried
stubbing them instead of spying on them, but this would have required
MockDataService class. This felt like tests were getting too intimate with
the production code and I decided not to stub.

So a good compromise seems to consider them integration tests. This makes
sense: the main() code actually “integrates” all the pieces, and doesn’t have a
lot of logic.
Firebase.watchOnlineStatus

Keep Firebase client in sync with the browser’s online status.
FirebaseAuthenticationService test: housekeeping
FirebaseDataService test: housekeeping