Improve leaving a DM experience

[Johennes]

We're trying to establish the concept of "one DM per user" on Element X. As an example, if a DM with a user already exists, trying to start a chat with that user will drop you into the existing DM rather than starting a new one. One edge case where this is problematic is when somebody leaves a DM.

We've identified the following cases:

• The other person left - I want to write again
  • Invite again
• The other person left - I want to delete the conversation (clean-up)
  • I can leave (if I am the only one)
• I want to leave a chat (clean up)
  • I should be able to leave
• I left and want to chat again
  • I should be able to create new room and end up in the old room

To support all of the above, let's do this:

• Enabling leaving DMs (via room details and the room list context menu)
• When sending a message into a DM room that contains no other user, spawn a pop-up that prompts to re-invite the other user

Localisations

Title: "You are alone in this chat"
Message: "Would you like to invite them back?"
Button 1: "Cancel"
Button 2: "Invite"