Feature/check date range

Adds endpoint /gt/service-dates to return start and end dates for feed service.

Validates that selected representative weekday and weekend are within the service date range.
I don't think falsy tests work like that in Java-land, but I could be wrong.  It should probably be using `isEmpty` instead of comparing to an empty string, though.  In any case, I don't think it will ever actually be an empty string here.