DEV: Allow specifying button class in reviewable action definitions (PR #8093)

This avoids the need for using @extend in SCSS, which can be problematic in plugins

See https://review.discourse.org/t/fix-make-compatible-with-debundled-plugin-css-assets-feature/5297/7 for context.

This is a draft to confirm API design. Tests will be added before merging.

GitHub

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