Improve rendering of Airbnb 'Call' buttons in inbox

Currently, when Airbnb adds a 'Call [host name]' button for a guest, Hospitable renders this in the inbox as a plain text message from the host (e.g., 'Call Vic And Joanna'). This makes it appear as though the host sent a cryptic or stray message to the guest.

The request is to render these as system events or call cards (e.g., 'Airbnb added a Call button for the guest') or to label them clearly so hosts can distinguish them from actual messages they have sent.