Accessibility: Labels the add users field

Fixes CNVS-13732

Test plan:
- Using a screen reader, visit the course roster.
- Open the add user dialog.
- Validate that the entry field for the email addresses is labeled.

Change-Id: I844152cedb452d0cb3030efbdb030196fc8c86b8
Reviewed-on: https://gerrit.instructure.com/36927
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Joel Hough <joel@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Aaron Cannon <acannon@instructure.com>