Send multiple images and descriptive text in one go via DM

[fpohlmann]

In https://element-io.atlassian.net/jira/servicedesk/projects/SUP/queues/custom/128/SUP-1031, the request was made for the ability to send multiple images and descriptive text in one go via DM. The argument is that sending multiple messages in sequence can be avoided and images that belong together can be sent at once. Another argument that is not mentioned in the ticket is that rate limits applied to message frequency can be avoided.