PLT-6890 Fix various scrolling issues (#6727)

* Fix various scrolling issues

* Move reaction scrolling to reaction list

* Handle scrolling when RHS opens

* Only run scroll update code when posts change

webapp/package.json

@@ -20,6 +20,7 @@
     "intl": "1.2.5",
     "jasny-bootstrap": "3.1.3",
     "jquery": "3.2.1",
+    "key-mirror": "1.0.1",
     "localforage": "1.5.0",
     "marked": "mattermost/marked#8f5902fff9bad793cd6c66e0c44002c9e79e1317",
     "match-at": "0.1.0",