Work out API versioning

[goto-bus-stop]

We'll need to decide on a versioning strategy. When üWave first started I think we were going for a /v1/-like URL-based approach. That seems fine, but we could also consider using headers. Either way, this needs to be worked out a bit more in terms of implementation as well.

Since #394 aims to change an existing endpoint's format, we should figure out versioning by then, so we can upgrade smoothly.

• the find-my-way router library supports versioning routes