Translations: Give dropdown of existing names from across the system

Translations: Give dropdown of existing names from across the system. Any translation not done yet or not done in the picked language should come in a dropdown.

Implies : don't allow them to translate any random string.

Fields to track for names:

- agency.txt: agency_name
- stops.txt: stop_name
- routes.txt: route_short_name, route_long_name
- trips.txt: trip_headsign, trip_short_name
- stop_times.txt: stop_headsign