Annotate query results with QuerySet instead of BaseManager

[cmsetzer]

In #3659, we discussed finding a better type annotation for Django ORM querysets than BaseManager.

I had the impression we'd need to add an external dependency for this, but it turns out we can in fact do QuerySet[LinkUser] and so on. Moreover, django-stubs seems to have its own nuances (and only supports Mypy?); seems not worth bringing in at present.

This branch swaps QuerySet type hints in for all our existing BaseManagers. There should be zero impact on Perma functionality, as this is purely a dev-side change.

[rebeccacremona]

Oh great!!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.05%. Comparing base (07d00eb) to head (b3ac30e).
Report is 3 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3668      +/-   ##
===========================================
+ Coverage    69.01%   69.05%   +0.03%     
===========================================
  Files           54       54              
  Lines         7478     7493      +15     
===========================================
+ Hits          5161     5174      +13     
- Misses        2317     2319       +2     

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