Add printable support to CSS

Summary:
Fixes T2146. This is a really simple approach, you just do:

  !print .rule {
    whatever: blah;
  }

And it transforms it into:

  .printable .rule {
    whatever: blah;
  }

  @media print {
    .rule {
      whatever: blah;
    }
  }

So we end up with these rules twice, but they should compress well and we shouldn't need that many of them, and this fix is way way simpler than all the nonsense I discussed in T2146.

Test Plan:
  - Added a unit test.
  - Added a simple rule to throw away the menubar when printing.
  - Checked the latter with `/?__print__=1`.

Reviewers: chad, btrahan

Reviewed By: chad

CC: chad, aran

Maniphest Tasks: T2146

Differential Revision: https://secure.phabricator.com/D7363