a11y and WCAG compliance

- [ ] Resolve PHPCS sniffs reported by new WCAG rules on the [a11y branch](https://github.com/gocodebox/lifterlms/tree/a11y). Most of these issues are related to inputs always having aria labels or actual labels and links always having aria labels or titles. There's maybe 50 issues identified here but all of them could be resolved pretty quickly.
- [ ] Run chrome lighthouse audits against LifterLMS screens (admin and frontend). Let's find the issues and record them as separate issues if they're not easy to resolve without a lot of rebuilding. Checking against some of what I imagined would be more problematic admin panel pages resulted in a small number of issues. It looks like most of our admin pages are around 80% compliant. Frontend I would imagine is more compliant and will require us to determine what is an issue caused by LifterLMS and what comes from the chosen theme. Running these on twenty- themes is probably a good idea as I would hope they're already pretty a11y friendly
- [ ] Ensure icons don't exist by themselves without a `.screen-reader-text` span
- [ ] Add wp.a11y.speak() for quizzes
- [ ] Add wp.a11y.speak() for notifications
- [ ] Keyboard navigation (focus on buttons, anchors, form elements
- [ ] Screen reader manual tests

wp a11y: https://make.wordpress.org/accessibility/handbook/best-practices/test-for-web-accessibility/test-for-web-accessibility-frontend-code/
wp.a11y.speak(): https://make.wordpress.org/accessibility/handbook/best-practices/markup/wp-a11y-speak/