Fix more unicode issues in diff parsing.

A bug came in about parsing hg diffs with unicode characters, which prompted me
to audit all of our scmtools. There were a bunch of issues lurking, most of
which were exposed when I added unit tests.

Testing done: Ran unit tests.

Fixes bug 3261.

Reviewed at https://reviews.reviewboard.org/r/5540/
Fix unicode and style issues in the markdown review UI.

The fix I did for markdown to properly handle unicode objects failed if the
object we were trying to iterate over was already encoded to utf-8. The
exception was caught and logged, but the resulting rendered view was empty.

I've changed the markdown UI to pass the error through to the user, so they at
least see something.

Once I got the file displaying, I noticed that there were a few issues with the
style rules. First, lists had their bullets overlapping with the line numbers
(because of the all-elements rule to get rid of padding). Second, we have a
bare 'h1' rule in common.less that made h1 smaller than h2. I've restored it to
the same size it should have been, but we should probably make the common.less
rule much more specific.

Testing done:
* Verified that without the encoding fix, I saw an error in the "Rendered" tab
  instead of nothing.
* Checked that with the encoding fix, I could see the page.
* Checked that the style changes fixed the issues I saw with this particular
  document.

Reviewed at https://reviews.reviewboard.org/r/5548/
User manual part 3: Update "Reviewing Diffs"

This change adds a bunch of new content for the "Reviewing Diffs" page to
explain several of our coolest diff viewer features. This also rewrites some of
the text to be clearer, and replaces all of the old screenshots with both 1x
and 2x screenshots of the modern UI.

Testing done:
Built the HTML manual and looked at all the pretty things.

Reviewed at https://reviews.reviewboard.org/r/5545/
User manual part 4: Reviewing Images and Reviewing Markdown Files pages.

This change makes some small text changes (mostly updating the notes about
replies), and adds pretty screenshots of each of the UIs.

Testing done: Built HTML docs.

Reviewed at https://reviews.reviewboard.org/r/5549/
User manual part 5: Replace screenshots on the "Issue Tracking" page.

Trivial replacement of screenshots with new versions, including @2x versions.
Merge branch 'release-2.0.x'