The wizard step-done icons are hard coded

Instead of using an icon theme icon, the wizard uses a hard coded checkmark. Similarly to the fact that we don't want hard coded X instead of crosses, we should replace this text checkmark with a call to an icon theme icon.

See https://github.com/xwiki/xwiki-platform/blob/a00e71e026b280cf5e738e3b60420e6d80f7522f/xwiki-platform-core/xwiki-platform-appwithinminutes/xwiki-platform-appwithinminutes-ui/src/main/resources/AppWithinMinutes/WizardStep.xml#L54