Add models for app configuration, peak travel times

These are basic models for storing the info we know we'll need. I
believe this covers what is explicitly specified in the wireframes and
functional spec, although it seems likely that we'll uncover other
things that need to be stored here as we create more indicator
calculations.
Add tests, model field validation.
Merge branch 'feature/app-config' into develop
Merge branch 'develop' of github.com:WorldBank-Transport/open-transit-indicators into develop