Hydra: cleaning up and expanding the jobsets API

Hey y’all, and in particular @lewo

I’ve asked @cole-h to work on brushing up the Hydra API a bit more to make the API more feature complete for jobsets and projects. To do that, it involves a little bit of API breakage: hydra-api: flesh out Jobset, JobsetInput schemas; implement DELETE /jobset/{project-id}/{jobset-id} by cole-h · Pull Request #937 · NixOS/hydra · GitHub specifically renaming a couple parameters.

Unless I hear back, I’m inclined to merge once there are passing tests. Does that look good to you?