fix iframe for external tool max width
fix iframe for external tool max width

fixes COMMS-537

test plan:
- create a course withan external tool with a selection_width >850px
  (1000px works well)
- create an assignment and associate it with the external tool
- view the assignment as a teacher or admin role
- notice that the right-side-wrapper containing the "Edit assignment
  settings" and "Speed Grader" buttons DO NOT overlap with the iframe

Change-Id: I97819d6faa9a6e02727ab9e72616db1b44411e26
Reviewed-on: https://gerrit.instructure.com/134150
Reviewed-by: Venk Natarajan <f9eab59b292314637a3c917fea4742741e36a19b@instructure.com>
Tested-by: Jenkins
QA-Review: Heath Hales <8ca81b68c7e2592ac614bd07f22fcc735ca55f93@instructure.com>
Product-Review: Felix Milea-Ciobanu <9384aa226ac9d3237ed0bc52e18ffcb251ee0bf7@instructure.com>

sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms