Add icon option for third level items in NavigationAccordion

[katydecorah]

This PR:

• adds an icon option for third level items. This option accepts a maki icon value.
• added test-case only css to set the max-width of NavigationAccordion to 400px (which is the max-width the component usually sees on the page) - this will help catch layout issues a little better.

Fixes #235

How to review

1. npm start
2. Navigate to /NavigationAccordion
3. Test Shows third level items with icons test case.

I also copied this component as a test in /mapbox-gl-js-docs:

1. Open /mapbox-gl-js-docs locally, pull down the nav-accordion-test branch.
2. yarn install
3. yarn start
4. Visit /mapbox-gl-js/style-spec/layers/#fill-extrusion to check the icons on the layer's page.

QA Checklist

• No errors logged to console.
• Accessibility score in Chrome DevTools Audit/Lighthouse is 100% with Lighthouse version: 5.7.0.
  • Score should be 91% due to NavigationDropdown, SectionedNavigation - Form elements do not have associated labels #248
• Component is accessible at mobile-size viewport.
• Component is accessible at tablet-size viewport.
• Component is accessible at laptop-size viewport.
• Component is accessible at big-monitor-size viewport.
• If applicable, create a PR in a site repo, copy the component, and test it. Push to staging and instruct the reviewer to test the component in-page.

Open the test cases app locally on:

• Chrome, no errors logged to console.
• Firefox, no errors logged to console.
• Safari, no errors logged to console.
• Edge, no errors logged to console.
• IE11, no errors logged to console.
  - [ ] Mobile Safari, no errors logged to console. not applicable
  - [ ] Android Chrome, no errors logged to console. not applicable

[katydecorah]

This is almost ready, I'm waiting for access to BrowserStack to test in IE.

[danswick]

Looks great to me! I tested in IE11 and things are rendering well and I'm not seeing any errors.