Removing z-index on icons

[dfmcphee]

WHY are these changes introduced?

Currently we apply a z-index to icons to allow for them to sit above the backdrop when using one. This causes strange issues in consumers because it means icons get an arbitrary z-index of 2, lifting them over things that they shouldn't be.

WHAT is this pull request doing?

This removes the z-indexes and instead changes the ::after used for the backdrop to be a ::before which naturally stacks correctly without a z-index.

How to 🎩

🖥 Local development instructions
🗒 General tophatting guidelines
📄 Changelog guidelines

Ideally any regressions here will be caught by percy, but poking around in the playground to see if you can find any holes in this approach would be useful.

🎩 checklist

• Tested on mobile
• Tested on multiple browsers
• Tested for accessibility
• [NA] Updated the component's README.md with documentation changes
• [NA] Tophatted documentation changes in the style guide
• [NA] For visual design changes, pinged one of @ HYPD, @ mirualves, @ sarahill, or @ ry5n to update the Polaris UI kit

[dleroux]

Nice!