WICKET-5557 Don't cache the result of AuthenticatedWebApplication#getWebSessionClass()

(cherry picked from commit ea68c8623196c7a91b9f7d8ce6b43f2a57013c42)
WICKET-5555 AbstractAutoCompleteBehavior.WrappedHeaderItem should depend on wicket-ajax.js

(cherry picked from commit c2691cb5de2e9aed4201dec020c0ebf01f6518fd)