Add release notes for Review Board 2.0 RC 2.

This contains the release notes for all changes made to 2.0 RC 2. It is,
fortunately, a smaller release than we're now used to.
Merge branch 'release-2.0.x'
Add a release date to the release notes.
Merge branch 'release-2.0.x'
Release Review Board 2.0 RC 2.
Merge branch 'release-2.0.x'