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/