tippyjs: Fix blueslip error on popover add emoji reaction hover.

This tippyjs event listener was active on 'add emoji reaction'
option in sender's popover menu of a message but it was only
intended for the add reaction button in message reactions bar
at bottom of a message.

Both of those having common selector `.reaction_button` caused
errors in tippyjs near `observer.observe` having wrong args.

Edited the css selector to be more specific to only target
add reaction button in reactions row of message.

This was introduced in 99e6f25.

static/js/tippyjs.js

@@ -53,7 +53,7 @@ export function initialize() {
     // message reaction tooltip showing who reacted.
     let observer;
     delegate("body", {
-        target: ".message_reaction, .reaction_button",
+        target: ".message_reaction, .message_reactions .reaction_button",
         placement: "bottom",
         onShow(instance) {
             const elem = $(instance.reference);