Add show deleted objects filter

[mjsir911]

Description

Code taken from #72, combined into a commit & tidied up a bit.

Related Issue

Fixes #72

Motivation and Context

Allows to see deleted objects from admin page.

How Has This Been Tested?

Manually (for now).

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have run the pre-commit run command to format and lint.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have added my name and/or github handle to AUTHORS.rst
• I have added my change to CHANGES.rst
• All new and existing tests passed.

Of note is that this is a joint PR, most of the code has been contributed by @marco-silva0000, @rhaver, & @duebbert.
I don't know if we'll need their explicit sign-off for this, but I figure the attribution would be good.

[codecov]

Codecov Report

Merging #1169 (e0015cc) into master (a768673) will decrease coverage by 1.04%.
The diff coverage is 48.14%.

@@            Coverage Diff             @@
##           master    #1169      +/-   ##
==========================================
- Coverage   97.30%   96.26%   -1.04%     
==========================================
  Files          23       23              
  Lines        1260     1286      +26     
  Branches      204      206       +2     
==========================================
+ Hits         1226     1238      +12     
- Misses         16       30      +14     
  Partials       18       18              

Files Changed	Coverage Δ
simple_history/admin.py	90.69% <48.14%> (-7.94%)	⬇️

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[mjsir911]

There's another peculiarity in this patchset that I'de like to see if I can find a work around for: the SimpleHistoryShowDeletedFilter.queryset() is returning a differently-typed queryset when enabled: HistoricalQuerySet[HistoricalChanges] when self.value() vs QuerySet[Model] when not.

Would there be any way to coerce a queryset of historical changes to a queryset of the underlying history_object?

Functionally, what I want is this:

queryset.model.history.filter(history_type='-').latest_of_each().values("history_object")

but this doesn't work since HistoricalChanges.history_object is a descriptor.

Additionally, since the database objects that we're filtering by are deleted anyways, would it be possible to get a queryset containing them? How .history_object works is by creating an ephemeral object in-memory, not sure if this will work with querysets, but I know django-simple-history already creates it own modified(mock?) querysets for things.