Move the "Extending Review Board" part of the manual up a level.

It doesn't really make sense to have this be part of the "User's Guide", since
ordinary users can't install or enable extensions. This moves the extensions
documentation up one level, to be a sibling of the "Web API Guide"
Show the close description on the diff viewer page.

The close description was only shown on the review request page,
not on the diff viewer page, despite showing the banner. This now
adds the close description there as well.

Fixes bug #2913
Merge branch 'release-1.7.x'