Allow mass emails with EmailMessage Object

[Muskan Vaswan]

Allowing sending in a list of email messages as arguments for send_mass_mail() function. This will allow the user to send multiple emails using the email message object, in a single connection usinf a simplistic syntax.

---

Example Usage:

messages = [
      EmailMessage(subject, message, sender, recipient, bcc, cc, reply_to=email)
      for subject, message, sender, recipient, bcc, cc, email in datatuple
  ]
send_mass_mail(messages=messages)

[Mariusz Felisiak]

The API for send_mass_mail() is ​frozen so we will not accept any new functionalities. Check also ​docs about sending multiple emails.