Adding react scrollbar to the RHS thread (#2693)

* Adding scrollbar to the RHS thread

* Fixing the RHS thread scrolling issue

webapp/package.json

@@ -22,6 +22,7 @@
     "perfect-scrollbar": "0.6.10",
     "react": "0.14.7",
     "react-bootstrap": "0.28.3",
+    "react-custom-scrollbars": "^4.0.0-beta.1",
     "react-dom": "0.14.7",
     "react-intl": "2.0.0-rc-1",
     "react-router": "2.0.1",