Disable use_socket manually for CUSTOMER13

This was causing tracebacks during message sending

(imported from commit 9dde67f07934fe7a5aef7036340eebb7c6f6a97a)

static/js/feature_flags.js

@@ -54,7 +54,7 @@ exports.mark_read_at_bottom = true;
 exports.propagate_topic_edits = true;
 exports.summarize_read_while_narrowed = false;
 exports.clicking_notification_causes_narrow = true;
-exports.use_socket = true;
+exports.use_socket = false;
 
 // Ready for deprecation.
 exports.collapsible = false;