Accessibility: Added labels to add feeds dialog

Fixes CNVS-13727

Test plan:
- Visit the external feeds dialog on the announcements page.
- Using a screen reader, verify that all fields are labeled.

Change-Id: I3613744e279dddaa0d99d9c9b9ddf50122b07ec4
Reviewed-on: https://gerrit.instructure.com/37090
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Joel Hough <joel@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Aaron Cannon <acannon@instructure.com>