Currently redirecting to an external URL through a permalink doesn’t
work because Rails raises a
`ActionController::Redirecting::UnsafeRedirectError` error.
This wasn’t the case before we upgraded to Rails 7.0.
This patch fixes the issue by using `allow_other_host: true` on the
redirect.
Limitations: the user profile "open external links in new tab setting" is
slightly broken for "External URL" permalinks.
Remove the copy from the admin permalinks page stating that this doesn't work.
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
