feat: personless User deletion via admin

[jennifer-richards]

This replaces the default UserAdmin with one that adds an action to delete selected User records that are not associated with a Person. A list filter is added to allow selecting / reviewing these records. Selecting all on a page will cause an option to "Select all users" to appear:

Clicking this will select all records, including those that are not on the current page of 100.

Selecting the action and clicking "Go" gives the usual review of objects / related objects to be deleted. It also applies two adjustments to the selected set. First, only users whose person field is null will be affected. Any others will be ignored. Second, the set of objects that will be is limited to 990. This is needed because Django has a global limit on the number of fields a request can contain (which is at its default of 1000). Clicking on the "Yes I'm sure" will carry out the deletion.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.75%. Comparing base (ea8377d) to head (8928dde).
⚠️ Report is 13 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9312   +/-   ##
=======================================
  Coverage   88.74%   88.75%           
=======================================
  Files         320      320           
  Lines       41674    41689   +15     
=======================================
+ Hits        36985    37001   +16     
+ Misses       4689     4688    -1     

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