#22 UI invitation
#21 #22 Domain class Invitation
#22 modification of UI list of Event Adding button invite
#21 Adding isUserAllowedToInvite() to EventService
#21 InvitationController action invite
#21 Moving isUserAllowedToEdit to InvitationService
#50 Test classes
#21 Add service invite() in InvitationService
#21 modification Invitation model javadoc