fix using the back button in the calendar

fixes CNVS-14475

test plan:
 - regress calendar functionality

 - from various places in canvas:
   - navigate to the calendar
   - change between calendar views (Week, Month, Agenda)
   - press back button to navigate to previous view
   - press back button to navigate to previous page (before calendar)

 - from anywhere in canvas
	 - navigate to the calendar
	 - change months several times
	 - press back button to navigate to previous page (before calendar)

Change-Id: I9b80864201b3c5215398fc703bb4fc16682938ee
Reviewed-on: https://gerrit.instructure.com/42611
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brad Horrocks <bhorrocks@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Matthew Wheeler <mwheeler@instructure.com>