fix default front page behavior

test plan:
 - on a new course (with draft state disabled)
 * click on 'Pages' in the left nav
   - shows edit form for a new 'Front Page'
 * enable draft state for the course
 * click on 'Pages' in the left nav
   - should indicate no pages have been created
   - should allow the creation of a new page

 - on a course (with draft state enabled)
 * navigate to an existing wiki page
	 * click 'Edit'
	 * click 'Cancel'
   - should be redirected to a view of the same page
 * navigate to a non-existent page (manually changing url)
   - shows the edit form for the page
   * click 'Cancel'
   - should be redirected to the pages index

additional test considerations:
 - this affects behaviors around the default page named 'Front Page' on
   new courses (specifically when draft state is disabled). testing of
   related behaviors should be tested (especially the api).

fixes CNVS-10269

Change-Id: I11c7b65b004922e76f680afdece1ddcb65d542e3
Reviewed-on: https://gerrit.instructure.com/28081
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>
Product-Review: Matt Goodwin <mattg@instructure.com>