Add support for image attachments to messaging client

This branch adds support for image attachments to the messaging client. The message size limit of 32 KiB applies to attachments, so this isn't suitable for real world use yet, but it will allow us to test the UI with small test images (some examples are attached to the ticket).

A limit of 10 attachments per message is imposed to ensure that the private message, which includes the list of attachments, fits within the size limit.

A private message does not depend on its attachments, allowing the message to be shown in the UI before all attachments are received.

Closes #1585 (closed)