Feature Request: Per-identity BCC field

On my work account, I'm using different identities depending on either I send e-mails or respond on behalf of my team or on my own. In both cases I use different BCC addresses, either to send the response to the team's mail alias, or to ensure that the message is signed with my own GPG key so that I could read it in the future.

At the moment K-9 allows only to specify a global BCC address, but not per-identity BCC address (CC is supported). This forces me to either not sent the e-mails from the phone at all, or to write the BCC address manually each time so that I sent it to the correct mailbox. If the BCC field could be set per-identity, that would make this process easier to use.

Start by locating the existing global BCC setting and the per-identity CC setting described in the issue, then trace how identities are selected when sending or responding. Done means each identity can specify its own BCC address and outgoing messages use the selected identity's address.