Consider priorities of Taskcluster tasks

[gsnedders]

Currently we run everything with the default priority ("lowest").

We might want to reconsider this, especially the balance between full runs and PR jobs, but we do need to be careful about task starvation.

What might be most worthwhile increasing the priority of are the decision and sink tasks — once they're queued, we really do want to run them soon, because they're relatively fast running gates.