removed padding from homework submission dialog

fixes CNVS-12247

test steps:
- add box as an LTI app
- create an assignment with file upload as a submission option
- log in as student and go to submit assignment
- select box (box.net)
- ensure that there is white padding around the contents of the dialog

note:
I couldn't get rid of the white padding on the very bottom of the dialog
window. This is acceptable white space.

Change-Id: I88eefae6516e5257cabbfb441a7dd64879634bd2
Reviewed-on: https://gerrit.instructure.com/32964
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Eric Berry <ericb@instructure.com>
Product-Review: Eric Berry <ericb@instructure.com>
QA-Review: Eric Berry <ericb@instructure.com>