change message students who modal to use em units
change message students who modal to use em units

fixes GRADE-1579

test plan:
 * Visit Gradebook
 * Open the "Message Students Who" modal for any assignment
 * Select "Scored less than"
 * Verify the grade input has a width in "em" units

Change-Id: I02e46e001ec3684f7d9761a335eda8f336a05943
Reviewed-on: https://gerrit.instructure.com/163769
Reviewed-by: Derek Bender <d24d8a7ee520e34b1cec38800225d9f36b552df7@instructure.com>
Reviewed-by: Adrian Packel <9c6a39807293376f29cba17542b4cb1b663ecc4a@instructure.com>
Tested-by: Jenkins
QA-Review: Derek Bender <d24d8a7ee520e34b1cec38800225d9f36b552df7@instructure.com>
Product-Review: Keith T. Garner <a1a80143a840c140f31c3495ebbd9ec906e09fed@instructure.com>
Reviewed-by: Keith T. Garner <a1a80143a840c140f31c3495ebbd9ec906e09fed@instructure.com>

sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms