user groups: Fix incorrect checking for open groups overlay.

Overlays.js incorrectly checked for whether `#groups` overlay
was open, so it re-assigned the global `open_overlay_name`
variable. As a consequence unwanted bluslip errors were thrown
when closing the groups overlay after any group event was
received while the `#groups` overlay was open.

static/js/overlays.js

@@ -40,7 +40,7 @@ export function streams_open() {
 }
 
 export function groups_open() {
-    return (open_overlay_name = "groups");
+    return open_overlay_name === "group_subscriptions";
 }
 
 export function lightbox_open() {