No way to review uploads to -proposed and -security using merge proposals

Hi,

The problem is this:

  * After release we don't upload to the release pocket any more in Ubuntu. We instead target things at
    -proposed and -security.
  * Currently those branches are only created as they are needed.
  * We have no way to request a review for code to be sent to them without them existing.
  * When they don't exist the diff is the same if we target the release pocket, but currently
    the permission checks mean that Ubuntu developers can't edit the merge proposal.

We could pre-create all these branches, but that would be further overhead at branch-distro time.

Another solution would be to have something like a merge proposal that proposed the addition of
an official branch, where you would be reviewing the changes in the proposed branch against
another pocket. The problem with this is that we don't, in general, want to make the contributors branch
the official branch, so it would make the detection of the completed state harder.

This has some overlap with a brief discussion I think we had a couple of times early on about
how to propose new packages.

Thanks,

James