RFC: advisory "branch_status" flag in branch.conf

[Martin Pool]

I'd like this.

I don't see why it needs to be colo-specific, so it shouldn't be named as such.

I think the Launchpad set of statuses is about right, except that I
think the best default is not "experimental" or "development" but
rather just unspecified, and this should probably just be given by not
setting the value, which will fit in nicely with existing branches.
If the user wants to set it, they can; pretending we know when we
don't is misinformation.

Martin