Fix flakey selenium specs for files

This change causes the files UI to wait
for all DOM content to be loaded before
initializing the router and listening
for LTI messages

flag=none

Test Plan:
Navigate to /courses/:id/files and verify
the Files UI loads and functions as before

Change-Id: Iee3b5579db889f455353184fd6c39a2716d42003
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/280800
Reviewed-by: Ahmad Amireh <ahmad@instructure.com>
QA-Review: Ahmad Amireh <ahmad@instructure.com>
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Product-Review: Weston Dransfield <wdransfield@instructure.com>

ui/features/files/index.js

@@ -18,6 +18,9 @@
 
 import router from './router'
 import {monitorLtiMessages} from '@canvas/lti/jquery/messages'
+import ready from '@instructure/ready'
 
-router.start()
-monitorLtiMessages()
+ready(() => {
+  router.start()
+  monitorLtiMessages()
+})