Combine all single-user chats

[CarlSinclair]

I think the main difference between mail (email) and messages (chat) is that mail allows you to have several different conversations with the same person. Messages/chat does not. It combines all your message history with this person into one conversation. This has become the default approach over time because it's a lot simpler for the end user and less confusing in case they need to go back and find an old message.

The mail module feels "old" in 2020 because it's more of a mail module than a messages/chat module. But with these recent updates, it's getting a lot closer to the latter. So maybe consider taking this final step? Or at least make it possible for a super admin to configure.

Supplemental suggestions for this feature:

• Remove the ability to add users to 1:1 chats
• New messages should open the existing conversation you have with the user, even if it's empty (Like Facebook). The modal should be reserved for creating group chats
• Rework the UI for 1:1 chats to remove the subject from the header and replace it with the photo and name of the recipient. 1:1 chats don't need a subject. Only group chats need a name (and icon. See #183).

[spoorun]

We agree fully with @Carl2213 's proposals here and their rationale.

One thing to add, email 'Subject lines' attached to a conversation aren't appropriate really, and don't work with the single=user chat mode: agree those should be removed. However, ability add an indicator of the subject for an email should exist, via (visible) tag. That gives the best of both worlds, so when a user receives a new message, or is scrolling through messages with another user, they can instantly see what the message relates to (if either user has chosen to attach a tag). It would also facilitate search.