ensure only pickable frontends can be returned

lib/pleroma/web/plugs/frontend_static.ex

@@ -78,7 +78,11 @@ def preferred_or_fallback(conn, :primary) do
         :primary
 
       frontend ->
-        frontend
+        if Enum.member?(Pleroma.Config.get([:frontends, :pickable], []), frontend) do
+          frontend
+        else
+          :primary
+        end
     end
   end