tippy: Fix blueslip error on edit history timestamp hover.

The timestamp in edit history ui also has .message_time.
We had a tippy hover event attached to .message_time
whose code assumes it to belong to an actual message
which caused an error on hovering the message timestamp
in edit history.

This commit fixes it by making the selector more specific.

static/js/tippyjs.js

@@ -147,7 +147,7 @@ export function initialize() {
     });
 
     delegate("body", {
-        target: ".message_time",
+        target: ".message_table .message_time",
         appendTo: () => document.body,
         onShow(instance) {
             const time_elem = $(instance.reference);