Don't disable all buttons on upload submissions
Don't disable all buttons on upload submissions

Test plan:
 - Create a file-upload assignment
 - As a student attempt to submit before attaching a file
 - You should receive an error message, but still
   be able to attach a file to the submission

Fixes EVAL-2066

flag=none

Change-Id: I866567c5a6887e8d0c388eb23310c0db14372cc0
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/278623
Reviewed-by: Adrian Packel <9c6a39807293376f29cba17542b4cb1b663ecc4a@instructure.com>
Reviewed-by: Dustin Cowles <25afa0fec7fcf03720748195b533ef56026fd0cf@instructure.com>
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
QA-Review: Dustin Cowles <25afa0fec7fcf03720748195b533ef56026fd0cf@instructure.com>
Product-Review: Jody Sailor

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