Accessibility: Fix and add to heading structure across Canvas

Fixes CNVS-14293

Test plan:
Visit the following pages and verify that each:
- has an unaltered visual appearance
- has exactly one level 1 heading which can be navigated to by screen readers
- When appropriate (I.E. when the length or organization of the page warrents
  it) has lower level headings which can be navigated to by screen readers
- when lower level headings are present, heading levels proceed in order, and
  logically

The following pages should be verified to confirm they meet the above criteria.
  Note that the URL fragments provided may not be reachable on all installs,
  and should be modified appropriately.
- /courses/1/assignments
- /courses/1/assignments/18
- /courses/1/discussion_topics (Sections should be level 2, individual
  discussions level 3.)
- /courses/1/users/prior
- /courses/1/pages/front-page
- /courses/1/pages/front-page/revisions
- /courses/1/assignments/syllabus
- /courses/1/outcomes
- /courses/1/rubrics
- /courses/1/quizzes/11/take?preview=1
- /courses/1/quizzes/11/moderate
- /courses/1/question_banks
- /courses/1/modules
- /courses/1/statistics
- /courses/1/confirm_action?event=conclude
- /courses/1/copy
- /courses/1/content_migrations
- /courses/1/content_exports
- /courses/1/content_exports
- /profile/communication
- /users/2/user_notes
- /courses/1/users/1/usage
- /courses/1/announcements
  - An additional heading can be seen by loading the page when no
    announcements exist.
- /courses/1/pages
- /courses/1/files
- /courses/1/quizzes
- /courses/1/settings
- /dashboard/files
- /courses/1/outcomes/users/1

Change-Id: I26d7dba115105c36026758cf55644139116224ec
Reviewed-on: https://gerrit.instructure.com/37669
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>
Reviewed-by: Ryan Florence <ryanf@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>