Accessibility: Labeled the available from and until fields on discussions

Fixes CNVS-13935

Test plan:
- Using a screen reader, create a new discussion topic.
- Navigate to the Available From and Until fields
- Validate that the screen reader reads the field labels correctly.
- Validate that the fields still opporate as expected.

Change-Id: I3f81dd13d8e4b04bb3034681e9fb17015e4fd9c5
Reviewed-on: https://gerrit.instructure.com/37842
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jayce Higgins <jhiggins@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Aaron Cannon <acannon@instructure.com>