Make credentials rules more specific

This was a very broad selector, so it does not belong in a specific
section like this. Do what all similar rules do and make it very
targeted.

app/styles/base.css

@@ -868,7 +868,8 @@ select:active {
 #noVNC_credentials_dlg.noVNC_open {
   transform: translateY(0);
 }
-.noVNC_hidden {
+#noVNC_username_block.noVNC_hidden,
+#noVNC_password_block.noVNC_hidden {
   display: none;
 }