only set front page when front page changed

test plan:
 - with draft state enabled
 * create two pages (A & B)
 * set page A as the front page
 * update page B (using the api)
   - update fields (e.g. title, body, etc)
   - include wiki_page[front_page]=false
 * navigate to ../pages
	 - page A should still be set as the front page

closes CNVS-11266
closes gh-405

Change-Id: Ifc22c715a36dc069430f6999c41128ea35b32c83
Reviewed-on: https://gerrit.instructure.com/30232
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>