add base_mail_bcc module

Allows to configure a list of email addresses to which a blind carbon
copy (BCC) of all emails is sent.

The BCC email addresses can be configured by the system parameter
mail.always_bcc_to as a comma-separated list of email addresses.
According to [1] this adds a Bcc header to the message, instead of replacing it. This may not be what you want to do.

[1] https://docs.python.org/2/library/email.message.html#email.message.Message.__setitem__