Handle combinatorial inputs better

[rickynils]

I commonly set up jobsets with the same inputs, tracking different branches. For example a project might depend on nixpkgs and the project's source repository. I'm interested in building for both the master and release-xx.yy branches of nixpkgs, and I probably have at least two branches of the project source. I might have more inputs that I also want to track.

This creates a combinatorial explosion of jobsets for a single project, with names like nixpkgs-master_proj-master, nixpkgs-master_proj-release, nixpkgs-release_proj-release etc.

With declarative Hydra projects, it is pretty convenient to generate these kinds of jobsets, but the UI experience is not nice. It would be lovely to have some kind of matrix view for the jobsets, maybe with support for dynamically selecting which combination of "input branches" you want to display.