Implement feature to have emails sent to admin

[AtaGowani]

Description

At the end of each email delivery round, an email should be sent to the admin with details on which emails were a success and which weren't.

[arborrow]

For birthdays, confirmation emails, (and eventually post retreat emails - currently not scheduled/implemented) it should indicate those who successfully received the email and those that failed and perhaps those that did not have a primary email address. For post retreat emails we will want to ensure that the group picture has been uploaded before sending the email.