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.
The 'TaskQueue' object is just a singleton @lossyrob wrote to keep our calls to future in a single place. The idea is to abstract out the computational context we rely upon - so that if, for instance, we later choose to use actor pools rather than global pools, it is trivial.

I'm not saying this won't work, I just think it makes sense to keep that pattern going. Perhaps the extent endpoint should be refactored; I haven't taken a look at it.