Notify Administrators users when an invitation is to be validated

When a user is invited, administrators must validate it before user is effectively created. New feature is about sending an email to the administrators (and power_user ?) group each time an invitation is sent.

Content of the email could be:
"You have a new user invitation to validate for XXX (email of the user to invite)." (with a link around "to validate" that redirects to the user registration tab of admin center.