hide URL box on module-item-edit for non-external items

also fix some style issues stemming from the button overhaul

fixes CNVS-17199

Change-Id: I62b1c762067d9ab6eb8bef53a74069ff2292e8b8
Reviewed-on: https://gerrit.instructure.com/45280
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Dan Minkevitch <dan@instructure.com>
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>