Fix issues with the extension documentation.

There were a few issues with the documentation on extending Review Board, which
this change fixes:

* The "Django apps" link was undefined.
* The "Django Evolution" link was pointing to a useless page.
* "fieldset_id" was being used instead of "field_id" in the
  ReviewRequestFieldsHook examples.

Reviewed at https://reviews.reviewboard.org/r/6167/
Remove propertyUtils.js from RB, now that we have the Djblets version.

Review Board was still using propertyUtils.js, even though Djblets
now bundles this for us. This caused the RB version ended up
overriding the Djblets version. The Djblets version is more featureful,
so that's a bad thing.

This removes it from the tree and the bundles.
Merge branch 'release-2.0.x'