Add Giving Tuesday fundraising banner (Fixes #15527)

[alexgibson]

One-line summary

Adds home page fundraising banner experiment for Giving Tuesday.

Issue / Bugzilla link

#15527

Testing

• http://localhost:8000/en-US/?v=a
• http://localhost:8000/en-US/?v=b
• http://localhost:8000/en-US/?v=c

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.83%. Comparing base (e775d32) to head (c0bce2f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15534      +/-   ##
==========================================
+ Coverage   78.82%   78.83%   +0.01%     
==========================================
  Files         158      158              
  Lines        8282     8287       +5     
==========================================
+ Hits         6528     6533       +5     
  Misses       1754     1754              

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