[components] Prevent fieldset grid blowout and fix related layout issues

[mariuslundgard]

Type of change (check at least one)

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation (fix or update to documentation)
• Maintenance
• Other, please describe:

Does this change require a documentation update? (Check one)

• Yes
• No

Current behavior

• Jørn reported that grid blowout still happens within PTE content.
• Discovered that it's caused by a case in CSS grids called "grid blowout" (see post by Chris Coyier).

Description

• Added grid-template-columns: minmax(0, 1fr); on the various places that form fields are spaced.
• While working on this (and testing throughly), I discovered several related layout issues.
• Fixed an issue in DefaultPreview where it would render an unstyled 0 when providing it with the property subtitle={0}.
• Added spotifyEmbed type for testing.

Note for release

• Prevent fieldset grid blowout and fix related layout issues

Checklist

• I have read the Contributing Guidelines
• The PR title includes a link to the relevant issue
• The PR title is appropriately formatted: [some-package] PR title (#123)
• The code is linted
• The test suite is passing
• Corresponding changes to the documentation have been made

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[bjoerge]

Great work!