All message recipients should be stored

[Weiko]

Scope, Context & implementation

Current implementation

The backend only stores the sender (using the header "from" value inside the message) in the MessageRecipient table. (Note: This is a bit counter-intuitive because the sender is not a recipient, we should probably rename this table)

Expected implementation

The backend should store all recipients of a message (use "to", "cc", "bcc", etc).

Note: We plan to add a new MessageThreadParticipant object in the future but it is NOT the same, a thread participant is an internal entity that represents an existing workspace-member that can be added to a thread (forwarded) without being an actual message recipient.