Fix promotions admin UI inconsistency

[mamhoff]

Summary

This improves the UI of the promotion edit page. The page now has an "activation" menu that allows for configuring either automatic, path-based, single-code or multiple-code activation, similar to how the legacy promotion system does it.

This PR also solves a bug with single-code promotions which previously could not be created without a persisted promotion.

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• [ ] I have written a thorough PR description.
• [ ] I have kept my commits small and atomic.
• [ ] I have used clear, explanatory commit messages.

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• 📖 I have updated the README to account for my changes.
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[codecov[bot]]

Codecov Report

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

Project coverage is 89.54%. Comparing base (2632096) to head (7c2b843). Report is 3 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #5936 +/- ## ======================================= Coverage 89.53% 89.54% ======================================= Files 782 782 Lines 17981 17987 +6 ======================================= + Hits 16100 16106 +6 Misses 1881 1881 ```

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