Fixed spacing on form dialog buttons

Adds some spacing between buttons for
dialog form modals.

fixes CNVS-16859

test plan:
* Enable "Better File Browsing" feature flag
* Go to any course's Files page
* Select a file or folder and click on
  "Restrict Access" button in the toolbar
  on top
* There should be spacing between the submit
  and cancel buttons
* Repeat for the "Move" dialog

Change-Id: If0fb977e1a61e2d8cb42d4ef74497a9b36f3344b
Reviewed-on: https://gerrit.instructure.com/44570
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Clay Diffrient <cdiffrient@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Product-Review: Dan Minkevitch <dan@instructure.com>