Allow users to link accounts for puppeting

[mymindstorm]

We can tell if a Matrix avatar changes easily since there is an event. I think you have the right event from Mumble's side.

Most of this would be authenticating and linking users. matrix-appservice-bridge lets us add data to a user context, so this would probably look like:

[Mumble] Authenticated User: !link @xxx:matrix.org
[Matrix] Bot DM to User: confirm Yes / No

http://matrix-org.github.io/matrix-appservice-bridge/1.11.1/MatrixUser.html

Originally posted by @mymindstorm in https://github.com/mymindstorm/matrix-appservice-mumble/issues/11#issuecomment-640817993