Allow Bcc to merchant on all email messages sent

[Traumflug]

This issue has come up here: https://forum.thirtybees.com/topic/1096/how-to-get-copy-sent-to-me-of-all-customer-order-related-emails and part of a solution is shown in this post by @dynambee https://forum.thirtybees.com/topic/1096/how-to-get-copy-sent-to-me-of-all-customer-order-related-emails/34

The point is that merchants get a copy of all sent email. For verification, debugging, archiving demanded by law, whatever.

Not put into a commit, yet, because there should be a Backoffice switch to turn this on and off.

[Dh42]

I think we need to merge with this issue since they are pretty much the same, #337

[Traumflug]

@wakabayashi made an override, which should be helpful for developing a permanent solution:

https://forum.thirtybees.com/topic/1798-1798/bcc-for-all-outgoing-emails/

[Traumflug]

Great work, @mke-tb ! Moved issue branch to 1.1.x after simplifying that code a bit (see commit message).

As always: if there's something left, please don't hesitate to re-open this issue or open another one.