Permissions: Updated guest user handling so additional roles apply

Previously additional roles would only partially apply (system or "all"
permissions). This aligns the query-handling of permissions so that
additional roles will be used for permission queries.

Adds migration to detach existing roles as a safety precaution since
this is likely to widen permissions in scenarios that the public user
has other roles assigned already.

For #1229

app/Permissions/PermissionApplicator.php

@@ -183,10 +183,6 @@ class PermissionApplicator
      */
     protected function getCurrentUserRoleIds(): array
     {
-        if (auth()->guest()) {
-            return [Role::getSystemRole('public')->id];
-        }
-
         return $this->currentUser()->roles->pluck('id')->values()->all();
     }