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/
Fix the revision selector on IE 9/10/11.

The code for the revision selector used a property for the page scroll
offset that wasn't standard across all browsers. That property is just
an alias for the actual standard property, pageXOffset. Switching to
this fixes the revision slider on IE.

Fixes bug #3373
Reviewed at https://reviews.reviewboard.org/r/6230/
Fix upload diffs on IE.

We were using File.readAsBinaryString() to get the contents of an
uploaded file. While Chrome and Firefox support this still, it's
actually a deprecated function that never made it out of beta, and IE
doesn't support it.

The proper solution, which is a bit more lengthy, is to read in as an
ArrayBuffer, create a Uint8Array view around it, and then convert the
contents to characters in the payload. This works on all browsers.

Fixes bug #3441.
Reviewed at https://reviews.reviewboard.org/r/6231/
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/
Release Review Board 1.7.28.
Fixed the syntax for a command in the 1.7.28 release notes.

The command was showing as a broken link, since in ReST, surrounding
text with single backticks makes it a link. We now use :command:`...`
instead.
Merge branch 'release-1.7.x' into release-2.0.x
Merge branch 'release-2.0.x'