Allow event registration notification email to multiple emails

Currently in CiviEvent doesn't allow multiple email addresses ( coma separated list ) in CC and BCC block for notification like we support in CiviContribute.

Steps to replcate :-

A. Under CiviEvent>> Manage Event >> Configure>> Online Registration if "Send Confirmation Email" is true
B. If we put multiple emails in CC or BCC block, It gives formrule error "Email is not valid."
C. Also fix help text accordingly.