Fixes#10180. When logged in as an unauthorized user who cannot access
the admin panel, the user is unable to log out through the prompted
`/admin/logout` page. This was because that page was using an incorrect
API endpoint, reading from `admin.user` instead of `user.collection`
when formatting the route. This page was also able to get stuck in an
infinite loading state when attempting to log out without any user at
all. Now, public users can properly log out and then back in with
another user who might have access. The messaging around this was also
misleading. Instead of displaying the "Unauthorized, you must be logged
in to make this request" message, we now display a new "Unauthorized,
this user does not have access to the admin panel" message for added
clarity.
