add sis_app_url Setting to PostGradesDialog

The URL to the SIS app where grades can be posted
for SIS integrations was accidentally hard-coded
in a previous patch set. This adds a Setting that
can be configured per Canvas installation.

Test Plan:
- Changing Setting.set('sis_app_url') to various
  values should make Post Grades work when it is
  set correctly and not work when it is not set
  correctly.

Fixes SIS-249

Change-Id: Id5d5659ace289771cf7adaa62429362039f1dc7a
Reviewed-on: https://gerrit.instructure.com/36360
Reviewed-by: Tyler Pickett <tpickett+gerrit@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
Product-Review: Eric Adams <eadams@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>