Editor: Editing URL after publishing

Allow users to republish a page under a new path. It needs to:

- [Delete](https://github.com/BeaconCMS/beacon/blob/cbe3bf7445cc559c96310593105e710366ed2cea/lib/beacon/router.ex#L204) the old page from the router
- Publish the new page version as usual
- ~Perform a 301 redirect from the older path to the new one~ extracted as a new feature, see https://github.com/BeaconCMS/beacon_live_admin/issues/97 so when editing a published URL it should generate a new record in the URL Redirect manager.

Start with the router deletion logic at lib/beacon/router.ex around line 204, then trace the normal page publishing flow and the URL Redirect manager. The 301 redirect work was extracted to Beacon Live Admin issue #97; done means republishing removes the old route, publishes the new path, and creates a redirect-manager record.

Written by the indexing model from the issue text.