fix(buttons): disabled IconButton background color

[jzempel]

Description

This PR is comprised of a bit of design rationale combined with dev rationale.

Design rationale

• Disabled icon buttons should reflect enabled icon buttons.
• Icon buttons don’t have a background when enabled, and they should not suddenly get a background when disabled. It would throw off our language around icon buttons if they suddenly got a heavy background.
• Garden icon buttons in split buttons do get a background, so they should still have a background when disabled.

Dev rationale

Since added state combinations are available on the code side, the design rationale is enhanced with:

• if the button isPrimary, we’ll retain the grey-200 disabled background
• if the button participates within any grouping (GroupButton or SplitButton), we’ll retain the grey-200 disabled background

Detail

Supports styling seen with zendeskgarden/ckeditor#5.

Checklist

• 👌 design updates are Garden Designer approved (add the
  designer as a reviewer)
• 🌐 demo is up-to-date (yarn start)
• ⬅️ renders as expected with reversed (RTL) direction
• 🤘 renders as expected with Bedrock CSS (?bedrock)
• ♿ analyzed via axe and evaluated using VoiceOver
• 💂‍♂️ includes new unit tests
• 📝 tested in Chrome, Firefox, Safari, Edge, and IE11

[coveralls]

Coverage increased (+0.003%) to 95.759% when pulling 5ff8238 on jzempel/disabled-icon-buttons into e5cd252 on main.