Keep to and cc of incoming mail
Implementation for #499.
This adds two options:
keep_incoming_cc: when enabled, this passes through the cc header of the original incoming email
keep_incoming_to: when enabled, this passes through the to header of the original incoming email
These features are disabled by default. When enabled, they allow the users in their clients to click on "Reply-All" and thus have their clients automatically set the headers correctly.
This option can be enabled, when usability is more important than having those headers encrypted. Note: Since these headers will be passed through when enabled, they are being sent unencrypted in the header.