don't attempt to render assignment index before DOM ready
don't attempt to render assignment index before DOM ready

if template streaming is enabled, it's possible that
assignment_index.js will start executing before the mount point
for the index (#course_home_content) is in the DOM.
wait for DOM ready before rendering the index

test plan:
 - ensure assignments index page works
 - ensure course home page works when set to assignment list

fixes LS-1406

Change-Id: Icb1b8dd9e4aec1639d42dbee4a919810af9d38b4
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/246340
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Alex Anderson <8690c85fe64134788579ce84d396a27368e019d5@instructure.com>
QA-Review: Robin Kuss <0b206b54367b2de0334896ea45826f0e3194bd35@instructure.com>
Product-Review: Jeremy Stanley <b3f594e10a9edcf5413cf1190121d45078c62290@instructure.com>

sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms