Fix and improve doc building for the RBTools manual.
Review Request #10034 — Created June 22, 2018 and submitted
Our custom Sphinx extension for RBTools was not compatible with modern
versions of Sphinx, due to a bad import. This is a pretty trivial fix by
itself, and one we've already done for Review Board, but this change
also goes a step further and adds some of the beanbag-docutils
extensions needed to enhance references and to link to integrations on
Built the docs. Checked that the integration link worked.