Add new cross reference options for the Review Board manual.

Review Request #8517 — Created Nov. 3, 2016 and submitted — Latest diff uploaded

Information

Review Board
release-2.5.x
5ac48a2...

Reviewers

These new cross references allow for linking to Django documentation
(using the version we use on readthedocs.org), Review Board source
files, Backbone.js class documentation, and content in the Djblets
documentation.

This will be used in the upcoming documentation changes.

Made use of this in some new documentation. Tested that all the
references worked correctly.