Properly set origin of fetch requests
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These changes aim to fix#15247
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix#15247 (github issue number if applicable).
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- [ ] There are tests for these changes
- [x] These changes do not require tests because cors is already tested with different origins
These changes require changes in tests, but I need help with that (see comments below).
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