Regression/e2e tests for HTTP API

[mitjat]

We have no (?) tests for the HTTP API, see e.g. #272

At a very minimum, we should add a simple script that curls some hand-picked URLs.

Even better would be to implement contract testing with an automated tool; we used dredd in parcel and it worked nicely.

Almost independent of that: We could have regression tests for a hand-picked set of URLs, where a script fails whenever those URLs' responses change. This is a nice tool for observing changes, but a little awkward because it requires a DB frozen at a chosen height. Probably not worth it.

Whatever we choose should be integrated into CI.