Implement backend for sending and receiving image attachments

Receiving Image Attachments

Private message headers will include a list of attachments, and there will be a MessagingManager method for loading an attachment, which will include the attachment data as a ByteBuffer, later to be converted to an InputStream if we find that loading images into ByteBuffers won't scale.

This implies that private message headers (in the sense of headers belonging to the private messaging client) will no longer correspond to the root class PrivateMessageHeader (from which all one-to-one headers currently inherit). The root class may need to be renamed, and some refactoring of the code that handles that hierarchy will be needed.

Sending Image Attachments

TODO: MessagingManager API for creating local messages with optional attachments