Remove legacy admin UI styling and clean up CSS definitions.
Review Request #10853 — Created Jan. 24, 2020 and submitted
This removes the old Django admin UI overrides styling and the old admin
UI definitions file. The Django overrides file is no longer necessary,
now that we control the styling for all the pages we really care about.
The definitions file had very little left in it, and its remaining
responsibilities have been moved into other files. Several other files
that were previously (but unnecessarily) including it have been updated
to include more specific files instead.
This is the last piece needed to allow packaging against Django 1.11.
All administration UI pages worked correctly.
Packaging succeeded on Django 1.11.