Add documentation on writing JavaScript extensions.

Review Request #8524 — Created Nov. 5, 2016 and submitted — Latest diff uploaded


Review Board


Something our Extending Review Board documentation has been missing for
quite some time is a guide on how to write JavaScript extensions, and
overviews of the (few) hooks we have available.

This change introduces these guides, providing examples of the various
ways that JavaScript extensions can be specified, configured, and
written, along with things to watch out for when writing these.

Built the docs and went through them, checking for errors, broken links,
and bad code samples.