Renamed "priority" github label to "release-blocker"

[Martin Pitt]

Hello all,

in practice we've been using the "priority" label as "must go into the next
release" for a while. Since our move of CI to AMQP the label actually hasn't
influenced the test queue priority; in principle we could implement that, but
so far we never bothered as we have enough capacity anyway.

So I renamed the label to "release-blocker", and also gave it a nice red tint:

https://github.com/cockpit-project/cockpit/labels/release-blocker

Martin