Fix issues around doc building.
Review Request #11147 — Created Aug. 20, 2020 and submitted
Documentation building had regressed during 2.0 development, largely due
to initialization differences in Django. Some modules were missing from
INSTALLED_APPS, causing a variety of code reference pages to break.
These have been added.
There were also some bad module names, and some missing/outdated extlink
Built the docs without errors. Checked that the output was as expected
for the pages that were broken.