Fix quote thumbnail flickering issue

Turns out that we reload thumbnails for every message when any new
message is added to the conversation. This fix prevents that by actually
checking for the proper sentinel on the message model

js/models/conversations.js

@@ -1250,7 +1250,7 @@
         // If we already have a quoted message, then we exit early. If we don't have it,
         //   then we'll continue to look again for an in-memory message to use. Why? This
         //   will enable us to scroll to it when the user clicks.
-        if (messages.quotedMessage) {
+        if (message.quotedMessage) {
           return;
         }