prevents overlapping of inputs in grade pass back dialog

fixes SIS-677

Test plan:
- go to the gradebook
- click the 'Post Grades' button
- resize the broswer window and confirm the inputs don't overlap

Change-Id: Ibf4d59aeb0c0a1374d71b19a204899de3d7d4982
Reviewed-on: https://gerrit.instructure.com/45328
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Tyler Pickett <tpickett+gerrit@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>
Product-Review: Cosme Salazar <cosme@instructure.com>