Bridges cannot determine if/when a user is deactived

Which means things like IRC connections / presence on the remote room is kept forever. We need a way to signal to appservices that a user has been deactivated. One idea could be to include some metadata on the leave event for the user OR sending a synthetic event to the AS,

Start by reviewing the specification's appservice behavior and leave-event definitions, then trace how user deactivation is represented today. Done means the specification defines an unambiguous signal that appservices and bridges can use to detect deactivation and remove remote presence.

Written by the indexing model from the issue text.