Redirects: extract :splat string into a constant (#12890)
## Summary
- Add SPLAT_PLACEHOLDER constant to redirects/constants.py
- Replace all :splat string literals in redirects/models.py and
redirects/validators.py with the constant
- Test fixtures in test files keep the literal string since they
represent user input
Per @humitos in the review of PR #11038: "It would be good to make this
:splat a constant."
Closes #11043
## Test plan
- [x] All 16 redirect tests pass
- [x] Pre-existing test failure in test_old_redirects.py is unrelated
(confirmed by running on main)
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