Allow adding or removing a list of users when sending a mail to a group

For example when considering the Send Page by Email use case, it would be interesting to allow users to specify one or several groups to send the mail to and also a list of users who will receive it.

Solution: Add the ability to specify a list of users to the GroupMimeMessageIterator implementation.

Note that the implementation should also check for duplicate users who would be both in the group and in the list of users so that they receive the mail only once.