postGradesInit() no longer dependent on @sections_enabled?

fixes SIS-323

TestPlan:
 - Enable Post Grades in account
 - Enable Post Grades in Course
 - verify Post Grades button renders in tabs
 - Post Grades should show dialog window when clicked.

Change-Id: If435b95bbdaf8a5d23ec803079934a631f7ed282
Reviewed-on: https://gerrit.instructure.com/36160
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Duane Johnson <duane@instructure.com>
Product-Review: Duane Johnson <duane@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>