better tinymce button spacing on assignment submit page

fixes CNVS-13615

test plan:
- create a text entry online assignment
- as a student, go to submit the assignment
- make sure the buttons are well spaced in the tinymce editor
- also try with with an assignment that supports multiple online types (so that
  the text entry tab is not the first to load)
- general regression of submitting an assignment - make sure an external tool
  tab (like google docs) still functions correctly.

Change-Id: I28b9895fc26a7319f41c8ba030cbd00d46d3256f
Reviewed-on: https://gerrit.instructure.com/36409
Reviewed-by: Cameron Sutter <csutter@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>