Add API explorer tool

[brainwane]

For the existing API endpoints, we could use https://readme.io/ or something like it to provide an API explorer tool -- an in-browser sandbox so a developer can interactively try out the provided endpoints and methods.

[miketheman]

I was thinking about this recently, along with other open issues about API documentation.

Considering using apispec to add documentation to API endpoints and then generate the UI based on the notations.

This would require a top-level route to serve the UI, or use an existing route and use content negotiation to serve if requesting HTML.