Where possible, adopt a subset of stylelint-config-wagtail changes.
- Formatting & ordering (auto applied), this includes the padding/inset shorthand changes
- Adopt a small set of no-union-classes changes as an example of changes to come in future PRs
- Ignore some areas where we are selecting against data* attributes that will not be practical to change
- Move some no-important ignore rules to specific lines
- Ignore max-combinators in modeladmin styles (legacy)
- Remove error messages forced-color-adjust setting to none
Ensures that when the iframe content is loaded there is a background on the iframe. So that transparent websites show correctly in the preview panel.
Fixes #11174
