Support for displaying emails in a thread (#659256)

Emails which are sent from ConSol CM when replying to or forwarding emails now include the correct message IDs in the In-Reply-To and References headers. Therefore, they are now displayed correctly in threads when using email clients like Microsoft Outlook, which rely on these headers for grouping emails in conversations.

This allows all participants in an email exchange to take advantage of the communication threads in their email clients instead of having to quote email content.