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.