Add new cross reference options for the Review Board manual.
Review Request #201 — Created Dec. 6, 2016 and updated — Latest diff uploaded
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. Testing Done: Made use of this in some new documentation. Tested that all the references worked correctly. Reviewed at https://reviews.reviewboard.org/r/8517/