A11y: Added labels to fields within the Modules Add Dialog

Fixes CNVS-15556

Test Plan:
- Visit the modules page.
- Create a module.
- Click the button to add content.
- Using a screen reader, verify that all form fields are labeled.
- Change the content type and verify all new fields that appear are also
  labeled.

Change-Id: Ia057a9203daa0a6146957da5d7e1b72adc79cb9e
Reviewed-on: https://gerrit.instructure.com/43507
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Clay Diffrient <cdiffrient@instructure.com>
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>
Product-Review: Aaron Cannon <acannon@instructure.com>