- Remove dead classes / dead code
- Move class definitions to where they are used, don't define in a
  shared space

Also, add documentation for alternate usage.

This allows us to make alterations to the generated routes based on the
scope of the current mapper, and otherwise allows us to move larger
blocks of concerns out of the routes file, altogether.

This pattern was introduced as a plugin by @dhh.

The original implementation can be found in
https://github.com/rails/routing_concerns