New milestones for Cabal

[Edward Z. Yang]

I've been trying to do triage on the bugtracker, and I've
been pretty unhappy with the old milestone system.  So I
tried something new:

    - No more separate Cabal/cabal-install milestones.
      We're tightly coupled, and if we do uncouple, then
      we can talk separate, but not while the projects live
      in the same repo.

    - Replaced the point and feature milestones with
      "Soon" and "Later".

      Soon = release manager (Mikhail) wants this ticket
      fixed in a patch release.  I think this is how milestones
      were currently being used in a useful manner today, so
      this basically should stay unchanged.  Only the RM
      gets to make decisions about what should happen soon.

      Later = some developer is planning to try to get this
      feature in to the next release cycle.  They're actively
      working on it, we can bug them when release time comes
      around for them to finish it up.  So this is NOT a bin
      where you just put things you'd like to see in the
      next release.  Either someone *is* working on it, or
      developers have agreed (e.g., via triage meeting/discussion)
      that this feature is a priority for the next release

    - I introduced a "Triaged" category for bugs which have
      repro'd test cases and just need someone to take care
      of them.  The idea is that if you feel like doing some
      bug hunting, just look for something in Triaged and
      fix it.  Not sure if this will work, giving it a try.

    - Everything else should be marked as Never.  That doesn't
      mean it won't happen, just that someone needs to specifically
      step up to make it happen.  I added a special subcategory
      for "Obsoleted by Nix build", since many bugs are in
      legacy code which won't manifest with the new-build
      path.

So, hope that helps turn the milestone system into something
a bit more useful.