Fetch legend text on translation change.
Return translated GTFS Route Type

Since the description was simply a CharField that was constructed upon
model initialization, it wasn't translatable. This returns the
human-readable string from gtfs.GTFSRouteTypes.CHOICES, which is static
and can therefore be translated.
Remove GTFSRouteType description field.
Make Angular respect language cookies on initial page load.
Merge branch 'feature/legend-translation' into develop