[EuiFormControlLayout] Fix prepend/append border radius in Amsterdam

[andreadelrio]

Summary

• Fix border-radius in EuiFormControlLayout in Amsterdam
• Add mixin to avoid code repetition
• Consolidated backgrounds to be the same color for all append/prepend items

Before

After

Checklist

• Check against all themes for compatibility in both light and dark modes
• Checked in mobile
• Checked in Chrome, Safari, Edge, and Firefox
  - [ ] Props have proper autodocs
  - [ ] Added documentation
  - [ ] Checked Code Sandbox works for the any docs examples
  - [ ] Added or updated jest tests
  - [ ] Checked for breaking changes and labeled appropriately
  - [ ] Checked for accessibility including keyboard-only and screenreader modes
• A changelog entry exists and is marked appropriately

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_4196/

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_4196/

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_4196/

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_4196/

[cchaos]

The contrast actually isn't great right now for buttons. But I'll update that in a follow up PR.

@miukimiu Can you take over the review since I was doing some of the updates too.

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_4196/

[elizabetdev]

The code looks good! 🎉

I looked at the code and tested in Safari, Firefox, Chrome, and Edge.

I just noticed that in Safari 14.0 and Firefox 82.0.2 (64-bit) the border/shadow is not looking 100% perfect.

But it seems that is an old issue that exists in master in all inputs.

The issue is on the box-shadow: 0 0,inset 0 0 0 1px rgba(19,34,149,0.1);. We can open another issue to solve this.

[cchaos]

Nice eagle eye! Yeah, it's not directly related to this PR. But yeah that 0 0, in the box-shadow is what is causing this. I'll go ahead and try to track down and fix.

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_4196/