UX: Move archive button in topic controls dropdown (mobile view). (#7681)

app/assets/javascripts/discourse/initializers/topic-footer-buttons.js.es6

@@ -130,6 +130,9 @@ export default {
         "archiveTitle",
         "toggleArchiveMessage"
       ],
+      dropdown() {
+        return this.site.mobileView;
+      },
       displayed() {
         return this.canArchive;
       }