Added Post Assignment to SIS

fixes SIS-214
fixes SIS-229
fixes SIS-230

TestPlan:
 - go to account settings and verify post grades feature is enabled
 - go to course settings and verify post grades feature is off
 - create a new assignment, verify "Post Assignment to SIS" checkbox is not present in the view
 - go to course settings and enable Post Grades
 - create a new assignment and verify Post Assignment to SIS is present in the view and checked. NOTE: it should be checked by default if this is a NEW assignment
 - save the assignment
 - edit the assignment again and the post grades checkbox should be true
 - uncheck the post assignment to sis checkbox, save
 - re-edit the assignment, the post grades checkbox should be unchecked

Conflicts:
	app/coffeescripts/models/Assignment.coffee

Change-Id: I34cba5add03b3cb935b502e47b4537bf93c5b7c1
Reviewed-on: https://gerrit.instructure.com/33411
Reviewed-by: Eric Adams <eadams@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>
Reviewed-by: Duane Johnson <duane@instructure.com>
Product-Review: Eric Adams <eadams@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>