Linear issue: [[ https://linear.app/comm/issue/ENG-160/send-keyserver-admin-message-on-report | ENG-160 ]], [[ https://linear.app/comm/issue/ENG-1700/create-endpoint-responder-for-message-report | ENG-1700 ]]. We need the keyserver to send a message report to the keyserver admin from the commbot user when a user
reports a message. This diff introduces functions for creating such report.