Running support: tickets, threading & replies — Chat and email in one thread

Where a reply goes is decided per ticket, not per channel. A ticket opened in chat is answered in chat; one opened by email is answered by email; one that started in chat and left an address gets both, so an agent types once and reaches the person wherever they actually are. The thread labels every message by how it travelled — Sent to customer for mail, a chat badge for a widget reply — because telling an agent their chat message went out as an email is the sort of small lie that costs trust.