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

Disable switch M24_WEBSITE_REFRESH

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 78.97%. Comparing base (2a4b476) to head (abc4cff).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #15534 +/- ## ========================================== + Coverage 78.95% 78.97% +0.01% ========================================== Files 158 158 Lines 8293 8298 +5 ========================================== + Hits 6548 6553 +5 Misses 1745 1745 ```

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