Make trackGit define declarative Hydra jobset

[Chris Warburton]

I'm still not quite sure how declarative Hydra jobsets work, but it
seems to involve a JSON file. We should have trackGit ask whether we
want to define such a declarative jobset.

If yes, we should define a suitably named jobset, containing two jobs:
one called foo-release and one called foo.

We should take this JSON, etc. from a template in ~/.templates.

The template should have a comment at the top telling us how to use it.
Since JSON doesn't allow comments, we may have to use a string inside
the object (as long as Hydra doesn't complain about extra attributes).

We need to make sure our release.nix template is suitable for use with
such a foo-release/foo split.