Zulip: Implement changing of group private chat titles

[Alpus]

Now, the titles of group private chats is just enumerations of user names and it is not convenient. It seems logically to add opportunity for each user to interchange that titles with more understandable for human. It might be implemented as button near the name like an arrow in the upper left corner of messages.

[timabbott]

This is a potentially interesting idea. I think probably the model I'd take for solving this problem is to have the ability to change a group private message conversation into a private stream, since that's effectively a nicely named group of users already. We'd need to be thoughtful about letting the user specify a range of messages to move over to the new stream, since a particular group of users might discuss different things over time and may not want to move their entire group history to a stream..