Jobtypes api

Another attempting at pushing the jobtypes API.

This one builds on versioned jobtypes from the recently merged jobtype_versioning branch.

This is not yet complete, only the endpoints without explicit version (which implicitly work on the newest version and create new versions on changes) are implemented, except for the code endpoint where this doesn't make sense.

All endpoints have been manually tested during development, but no thorough unit tests exist yet.