🐛 Fixed 404 handling in admin
🐛 Fixed 404 handling in admin

refs https://github.com/TryGhost/Ghost/issues/9571
- use `replaceWith` rather than `transitionTo` so that the back button still works and doesn't immediately throw you back to the 404 page
- use the new `router` service to avoid deprecation notices
- update the regex replacements to work with `#` urls

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