Add BCC to outgoing e-mails

[YetAnotherGerrit]

With this addition for our outgoing mails we can support situations where other people should not every e-email address because of data privacy reasons.

As an agent, I want to send e-mails with recipients in the Bcc field.

As an admin, I want to enable/disable the Bcc field.

As an agent, I want to see the recipients of the Bcc field for outgoing e-mail articles.

As a customer, I should not see the recipients of the Bcc field of e-mail articles.

Use-Case

An agent from a school receives mails from different sources sometimes, that need to be forwarded to parents of a class. The parents of the class should not see each others e-mail addresses due to data privacy requirements.

Acceptance criteria

• Should be configurable (the default should be disabled) inside the UI.
• Possibility to add Bcc on all outgoing mails.
• A customer should not see Bcc information.
• The agent should see Bcc information after sending.
• The existing system_bcc (already existing hidden feature) recipients should always be added on top.

Relevant Ticket:

Ticket#10173348, #10187416

[MrGeneration]

Honestly, I'm not a fan of all these hidden settings. There are super useful settings we hide that users never find, because they don't read the documentation. BCC field has been requested quite often and we're planning on hiding it...?

(I am suprised we "suddendly" do want to add this after telling people to gtfo so long, but that's just me)