Allow running save_routes on historic GTFS files

[EddyIonescu]

Currently we only store the latest version of route-configs, but have versioning for timetables.

save_routes.py runs daily and updates these using the recentmost GTFS that is available.

Using TransitFeeds (https://transitfeeds.com/api/swagger/) we can run save_routes.py on older versions of the GTFS based on the start/end dates of each GTFS file. Then we'll be able to show scheduled times and the proper routes for dates in the past.

For the most part, the agency config can be changed while remaining compatible with previous versions of the agency's network. However, for cases like where a branch is completely changed or the routes are redesigned, a different version would be required here too (see #127), which can be done in another issue and PR.