Add release notes for 2.0 RC 3.

This covers all the fixes and improvements made in 2.0 RC 3. There's a
number of bug fixes, performance improvements, extensibility
improvements, and a couple experimental Web Hook additions in here.

Reviewed at https://reviews.reviewboard.org/r/5758/
Merge branch 'release-2.0.x'
Add input escaping to the bugs section of review

If you create a new review and type a string that should be escaped into the
bugs secion (e.g. <A>), it disappears. Can be fixed by using view.formatText
that escapes input. Note that even though the text that you entered isn't
displayed, if you refresh the review, it's there, and is displayed, as the
template has "|safe" in it. The problem is only visible after immediate value
update.

There are a couple of other fields that might have the same issue, but none of
them (depends on, target groups) are likely to have input that should be
escaped in them.

Testing Done:
Before:
1) Created a new review.
2) Click edit on bug description.
3) Enter "<A>" into the field and press enter.
4) Text disappears.

After:
1) Created a new review.
2) Click edit on bug description.
3) Enter "<A>" into the field and press enter.
4) Text is there, just as expected.

Also tested behaviour with multiple inputs like "<123> <456>".

Bugs closed: 3244

Reviewed at https://reviews.reviewboard.org/r/5774/
Make a small improvement to unicode diffs in 1.7.x

Generally, Review Board 2.0's support for unicode is much, much better than
1.7.x, but this is a small enough change that it's probably worth making. Some
of the filename processing can do operations which will fail if the filenames
an un-decoded utf8, so convert all filenames from diffs to unicode if the
SCMTool didn't already.

Testing done: Ran unit tests.

Reviewed at https://reviews.reviewboard.org/r/5784/
Merge branch 'release-1.7.x' into release-2.0.x
Update the 2.0 RC 3 release notes for the Djblets changes.

The release notes were lacking some of the more key changes made to
Djblets. This updates them to include those changes as well.
Release Review Board 2.0 RC 3.
Merge branch 'release-2.0.x'