hide Post Grades for non-SIS contexts

The "Post Grades" button should be hidden for courses and sections
that do not have an integration_id, and visible when they do.

Test Plan:
- Give a section an integration_id and it should show the
  Post Grades button when selected

Fixes SIS-333

Change-Id: I31fdc25db0ee58a528f5c4010fdab70f132a1998
Reviewed-on: https://gerrit.instructure.com/36768
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Tyler Pickett <tpickett+gerrit@instructure.com>
Reviewed-by: Eric Adams <eadams@instructure.com>
Product-Review: Eric Adams <eadams@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>