HTTP REST API infrastructure - fixes 886

Fixes #886.

Code is still camelCaps, we can fix that later sometime. Some code is still unported, in particular the Sphinx extension; we can do that when we get to writing documentation.
`userDocumentation`? :smile:   As an example for future API endpoints, it makes sense to do that here.  Though I admit there's not much need for user-facing documentation for this API...