Add a UI for allowed redirect domains

Whitelisted redirect domains are defined via a filter right now. It'd be useful to have a UI for this to remove the need for developer involvement to unblock marketing teams.

As an aside we could look at filtering the allowed domains just in time based on the redirect target. From a security standpoint if a bad actor is able to create a redirect they would presumably be able to update a stored domain whitelist as well unless we create a custom permission for admins / super admins only.

Start by locating the filter that currently defines whitelisted redirect domains and trace how redirect targets and permissions are handled. Clarify the proposed UI, storage behavior, and admin or super-admin access rules before implementing; done should include a usable domain-management interface and agreed security behavior.

Written by the indexing model from the issue text.