Updates to work process

I have made some tweaks to the issue list labels and milestones to assist
with the issue triage.

Mostly these are cosmetic but may change how you work.

I have combined some of our custom labels with prefixes in the same fashion
as we have document "Doc:*" prefixes. These are categories "Cat:*",
horizontal review "HR:*", and priority "Priority: *" under some .

Have moved the priority to use labels and not milestones as these are often
changing and we should use milestones to set dates against issue which
indicate when we plan to deal with those issues (same as we did in the WG
repo).

All issues now have a priority. It was a first pass by me and I'm happy to
get feedback that the WG wants to change the priority.

The chairs will spend some time this week to define what we believe should
be the priorities of the WG for discussion on Thursday and will then
re-look at the issue to match this high level set of priorities.

I will update the wiki to reflect this later today.

Adrian