minor: Use page_params.is_realm_admin.

We don't need to get this off the sub.

static/js/stream_popover.js

@@ -245,7 +245,7 @@ function build_topic_popover(opts) {
         topic_name,
         can_mute_topic,
         can_unmute_topic,
-        is_realm_admin: sub.is_realm_admin,
+        is_realm_admin: page_params.is_realm_admin,
         color: sub.color,
         has_starred_messages,
     });