Optimize assorted Django operations

[aviupadhyayula]

Meant to generally optimize our usage of the Django ORM, with a focus on taking full advantage of major upgrades in #605.

Changelog:

• Use subqueries to update membership & favorite counts in update_club_counts. Shows a ~70% speedup in local benchmarks (n=2000 memberships/favorites).
• Use bulk updates when updating decision reasons for application submissions.
• Use bulk create when creating activities fairs for registered clubs.
  • Concerned with how bulk_create plays with AutoField primary keys (like we have for the Events model). But, the documentation mentions that pSQL should be fine.[1]
  • I'm assuming the order of the returned Events objects doesn't matter. Would appreciate a second set of eyes.
• Use qs.exists() to check existence instead of qs.first() is not None whenever possible.
• Avoid fetching objects from DB into memory whenever possible.

[1] https://docs.djangoproject.com/en/5.0/ref/models/querysets/#bulk-create

[codecov]

Codecov Report

Attention: Patch coverage is 48.85496% with 67 lines in your changes are missing coverage. Please review.

Project coverage is 69.29%. Comparing base (813cd4b) to head (c5a912d).
Report is 20 commits behind head on master.

Files	Patch %	Lines
backend/clubs/views.py	44.82%	64 Missing ⚠️
...nd/clubs/management/commands/update_club_counts.py	25.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #627      +/-   ##
==========================================
+ Coverage   69.11%   69.29%   +0.17%     
==========================================
  Files          30       30              
  Lines        6362     6353       -9     
==========================================
+ Hits         4397     4402       +5     
+ Misses       1965     1951      -14     

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