Add a deny option for the proxy default role (#24145)

* backend

* frontend

* docs

* fix none default role casing and name reserved roles in the error

* reserve every casing of none as a role name

web/public/locales/en/config/global.json

@@ -212,7 +212,7 @@
     },
     "default_role": {
       "label": "Default role",
-      "description": "Default role assigned to proxy-authenticated users when no role mapping applies."
+      "description": "Default role assigned to proxy-authenticated users when no role mapping applies. Set to 'none' to deny access to unmapped users."
     },
     "separator": {
       "label": "Separator character",

web/public/locales/en/views/settings.json

@@ -1738,7 +1738,8 @@
     },
     "defaultRole": {
       "admin": "Admin",
-      "viewer": "Viewer"
+      "viewer": "Viewer",
+      "none": "None (deny access)"
     }
   },
   "globalConfig": {