Use i18n property files for CSS class names

"Until Wicket 7.0.0 some components have been setting hardcoded CSS class names (e.g. FeedbackPanel), other were using a overridable Behavior to set the needed class names (e.g. OrderByLink's ICssProvider).

Since Wicket 7.0.0 this is simplified and now the applications can provide their own CSS class names when needed by overriding some keys in their i18n properties files (e.g. in MyApplication.properties)."

Impact: small. Not sure if we override CSS class names somewhere. If so we could leverage this new feature.