TinyMCE: Added meaningful labels to the title and heading of the help dialog

Fixes CNVS-9796

Test plan:
- Launch a TinyMCE editor.
- With focus in the editor, press ALT+0 to open the Accessibility Help dialog.
- Using a screen reader, verify that the dialog title and first heading are
  meaningful and accurate.

Change-Id: Ibc1b9d61124686d69ef3e268af02b879c17c0c1c
Reviewed-on: https://gerrit.instructure.com/28443
Reviewed-by: Derek DeVries <ddevries@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Aaron Cannon <acannon@instructure.com>