Administrators should not be able to send several user invitations with same username to several different email addresses

How to reproduce:

• go to ADMIN > Users & group
• create a new user with username invitee1 and email address invitee1@nuxeo.com, click invite button
• create a new user with the same username invitee1 and different email address invitee2@nuxeo.com, click invite button

Second invitation should give an error as an invitation has already been sent for username invitee1