a11y: Removes duplicate header from wiki pages

Removes the SR-only header and replaces the h2 page title
with an h1 page title (maintaining the same style) which
keeps a duplicate header from being read by SR.

fixes CNVS-16686

Test Plan:
  - Go to a wiki page
  - Screenreader should only have the page title header
    detected once, as a H1.

Change-Id: Ie4f4bb1bd45b50ae62aeb91e4df50392c0e73cec
Reviewed-on: https://gerrit.instructure.com/43854
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Dan Minkevitch <dan@instructure.com>
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>
Product-Review: Dan Minkevitch <dan@instructure.com>