Add support for Gitter users to start DM conversations with Matrix users

Add support for Gitter users to start DM conversations with Matrix users

Fix #2671 (closed)

Matrix DM support was first added in !2134 (merged)

• Adds Chat privately button to Matrix virtualUser popover. This will redirect you to the Matrix DM room
• Creates Matrix DM room on demand when you visit the DM URL, ex. /matrix/5f762f95986e461e663059d1/@root:my.matrix.host
  • There are checks in place so you can't create a DM for someone else by visiting their URL
  • You also can't start a DM conversation with a Matrix user from the gitter.im homeserver because Gitter users should just chat to each other on Gitter.
• If the Matrix user leaves the DM room, when the Gitter user sends another message, the Matrix user will be invited back to the DM room (inviteMatrixUserToDmRoomIfNeeded)
• If the Gitter user leaves the DM room, when the Matrix user sends another message, the Gitter user will be joined back to the DM room (inviteGitterUserToDmRoomIfNeeded)

Dev notes

• server/handlers/app/index.js#L76-93
• modules/rooms/lib/room-context-service.js
• modules/rooms/lib/uri-resolver.js

Todo

• Upload avatar
  • On Gitter, we only have group avatars so they can't be individualized
  • On Element, it already picks up the avatar
• Make sure that if Matrix user leaves, starts DM with same user, it all works in the same room
  • Add test
• When Gitter user leaves, they can come back to the same room and join again
  • Add test
• When Gitter user has left the room, is invited back when the Matrix user sends another message
  • Add test
• When Matrix user has left the Matrix room, is invited back to the room when the Gitter user sends another message
  • Add test
• Ensure that you can't initiate the DM room conversation by visiting someone else's URL
  • Add test
• 404 when you visit a URL for a Matrix user that does not exist, http://localhost:5000/matrix/5f762f95986e461e663059d1/@this-person-does-not-exist:my.matrix.host
  • Add test
• Block starting a DM from Gitter for any user from the gitter.im homeserver. I can imagine some weird message loops if we allowed this
  • Add test