add auto width to fill in empty space after Start and End Date fields
add auto width to fill in empty space after Start and End Date fields

closes EVAL-1270
flag=none

Test Plan:
1.  Navigate to Gradebook History view
2.  Confirm width for "Start Date" and "End Date" fields

Change-Id: I7572dc9fe3a233733a921f6ecd4a7244272a4398
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/258007
Reviewed-by: Spencer Olson <7cc0890d6f1f70810a023e7173eca5724d6c8e49@instructure.com>
Reviewed-by: Gary Mei <4dff5ebcff9ed76793b70b9470ac64791a16ed11@instructure.com>
QA-Review: Kai Bjorkman <4693b3d9e71784d629732770ab5e83ed8fbdb83e@instructure.com>
Product-Review: Syed Hussain <1379c89cd1d253d3c5a299cc4d8f6002ada9d6b5@instructure.com>
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>

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