Bring back submission selection triangle
Bring back submission selection triangle

fixes GRADE-1864

Test plan:
- Have an assignment with online submissions
- Submit multiple things for the same student
- View that student in SpeedGrader in various browsers, including
  Firefox and Edge
- In every browser, the submission selection dropdown should have
  a selection arrow/triangle on the right-hand side

Change-Id: Ib50a7e3a220416c2d7803475a5ed8da130473fd0
Reviewed-on: https://gerrit.instructure.com/176576
Tested-by: Jenkins
Reviewed-by: Derek Bender <d24d8a7ee520e34b1cec38800225d9f36b552df7@instructure.com>
QA-Review: Derek Bender <d24d8a7ee520e34b1cec38800225d9f36b552df7@instructure.com>
Product-Review: Jonathan Fenton <5549b92428699b688d6c4f80786628c27e48a3e1@instructure.com>

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