Pull routing out of us-forms and into forms repo

[easherma]

Stubbing this out to fill in detail later, but we've got a decent plan of action for addressing the fact that us-forms-system uses an old version of react-router: based on USDS advice we can pull routing out of us-forms-system and move routing into forms.

This makes more sense to try now that our forms are consolidated, and can be a step towards integrating forms into janis/alpha.

Relevant/related issues:
usds/us-forms-system#168 (comment)
cityofaustin/janis#269 (Essay by Mateo)
usds/us-forms-system#257
usds/us-forms-system-starter-app#27