Add missing button classes

[Etheryte]

What does this PR change?

Don's video highlighted a UI bug, this button doesn't have the regular button classes.

GUI diff

Bugfix.

• DONE

Documentation

• No documentation needed: Bugfix.

• DONE

Test coverage

• No tests: already covered

• DONE

Links

• DONE

Changelogs

Make sure the changelogs entries you are adding are compliant with https://github.com/uyuni-project/uyuni/wiki/Contributing#changelogs and https://github.com/uyuni-project/uyuni/wiki/Contributing#uyuni-projectuyuni-repository

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Re-run a test

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• Re-run test "changelog_test"
• Re-run test "backend_unittests_pgsql"
• Re-run test "java_pgsql_tests"
• Re-run test "schema_migration_test_pgsql"
• Re-run test "susemanager_unittests"
• Re-run test "javascript_lint"
• Re-run test "spacecmd_unittests"