Backport FileDiff migration and condensediffs to 1.7.x.

While Review Board 1.7.x introduced FileDiffData for new diffs, it never
gained the migration support. This was added for 2.0, by way of
on-access migration in FileDiff and the condensediffs management
command.

Diff migration has been thoroughly tested at this point, and is known to
work really well. It's also a beneficial thing to run before an upgrade
to 2.0. This change backports everything needed to get modern
(pre-RawFileDiffData) migration in 1.7.x.

Reviewed at https://reviews.reviewboard.org/r/6229/
Add release notes for 1.7.28

This covers all fixes and improvements since 1.7.27.

Testing done: Built html docs and looked at them.

Reviewed at https://reviews.reviewboard.org/r/6251/