Add attachment API for sending/retreiving private messages

This MR refactors PrivateMessageHeader to a ConversationMessageHeader base-class and adds Attachments to PrivateMessageHeaders.

It includes several commits that can be split up into individual MRs if needed.

While this is for a yet non-existent feature, this code should be safe to merge already to provide the API to build backend and UI on.

Part of #1434 (closed)