Explicitly move `/api` to `/api/v1`

We're currently providing two API versions:

https://github.com/Duke-GCB/bespin-api/blob/36650d8f067838f5bf54c833596cf7ce2955b4bd/bespin/urls.py#L7-L8

`/api/v2` is used by [bespin-cli](https://github.com/Duke-GCB/bespin-cli) and `/api/` is used by [bespin-ui](https://github.com/duke-gcb/bespin-ui/) as well as some internal machinery like [lando](https://github.com/duke-gcb/lando/) and [bespin-mailer](https://github.com/duke-gcb/bespin-mailer/).

At the very least, we should add an `/api/v1` entry for consistency, and update the rest of the clients to use this path (or v2 if possible). Ultimately we should be able to remove the unversioned `/api` path

See https://github.com/Duke-GCB/bespin-cli/issues/29