make the name input label visible in new assignment view
make the name input label visible in new assignment view

fixes EVAL-2295
flag=none

test plan:
- have a course with atleast 1 teacher.
- go to the assignments view.
- click on the "Add Assignment" button.
- notice the assignment name input has a label above it.
- notice the assignment name input has no placeholder
  text.

Change-Id: Ib836d2df38e6d1cdee9940dd5bf71e6207f3af90
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/287446
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Product-Review: Jody Sailor
QA-Review: Kai Bjorkman <4693b3d9e71784d629732770ab5e83ed8fbdb83e@instructure.com>
Reviewed-by: Aaron Shafovaloff <d0ee6b99167c663aba7bc3efd806fe8d6cad437b@instructure.com>
Reviewed-by: Dustin Cowles <25afa0fec7fcf03720748195b533ef56026fd0cf@instructure.com>

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