Add gl-button classes to admin/appearances

What does this MR do?

This adds the gl-button classes to the admin/appearances haml views as part of our effort to remove bootstrap and jQuery

#231195 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content