a11y improvements to the super-rentals tutorial

[jenweber]

I am proposing some a11y-inspired styling changes and additions to the super-rentals app, and need to propagate the changes to the tutorial.

Sister issue to super-rentals ember-learn/super-rentals#86
Related PR in super-rentals ember-learn/super-rentals#87

Checklist from Super Rentals:

• a11yAudit test inclusion not explaining in the tutorial, but adding internal tests to super-rentals
• installing ember-a11y and ember-a11y-testing deps skipping, just installing testing w/documentation in the super-rentals app
• alt text in hbs-helper.md, service.md, simple-component.md, subroutes.md
• input label
• focusing-outlet skipping, just going to link to the ember-a11y project
• styling changes into https://github.com/ember-learn/ember-cli-tutorial-style

Skipping:

• very minor changes to colors and font sizes to address contrast issues

[jenweber]

Discussed w/Mel and Locks: including a11y testing in the tutorial itself is maybe too much until we have dedicated beginner/intermediate tutorials. In the interest of not confusing new devs, I'm walking some changes back.