New draft statuses and magic branches

Adds draft status to Change. DRAFT status in change occurs before NEW
and will be for a change that is not meant for review.
Also adds magic branches refs/draft/ and refs/publish/ that
will handle whether or not a patchset is a draft or goes straight to
review. refs/for/ should be deprecated in favor of explicitly marking
a patchset as a draft or directly to review.
For now, the draft flag and status are unused and refs/draft and
refs/publish just mimic refs/for's functionality.

Change-Id: I95cb336f1944552f842f0b99dc299eb51e586801
4 files changed