Accessibility fixes and tests.

common/test/acceptance/pages/lms/course_wiki.py

common/test/acceptance/tests/lms/test_lms.py

lms/static/js/wiki/CodeMirror.init.js

lms/static/sass/course/wiki/_wiki.scss

lms/templates/wiki/base.html

lms/templates/wiki/edit.html

lms/templates/wiki/history.html

lms/templates/wiki/includes/breadcrumbs.html

lms/templates/wiki/includes/cheatsheet.html

lms/templates/wiki/includes/editor.html

package.json