Makes the feature flags accessible

Adjusts the element_toggler to use a span rather than a link.
Also adds a tabindex to the description so that screenreaders can
actually read the description.

fixes CNVS-14484

test plan:
  - Using a screenreader go to account settings (/accounts/#/settings)
  - Navigate to the Feature Options tab
  - Navigate to the toggle feature details button
  - Using either spacebar or enter should cause it to activate
  - Tabbing through should read the details if they are expanded

Change-Id: I50295fdcee91f90884383a340911ed8b8778deae
Reviewed-on: https://gerrit.instructure.com/39297
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Hilary Scharton <hilary@instructure.com>