Close live Action Cable connections on sign out (#268)

Action Cable authorizes a Connection once at the WebSocket handshake and
never re-checks it. Destroying the session record refuses future handshakes
and HTTP requests bearing the cookie, but a socket opened before sign out
keeps its handshake-time current_user and keeps authorizing new
subscriptions and delivering frames as the signed-out user.

Reset the user's remote connections when the session is terminated. Clients
tear down and reconnect: the signed-out device carries a destroyed session
and cleared cookie and is rejected at the fresh handshake, while the user's
other devices with still-valid sessions reconnect and stay live. This reuses
the existing reset_remote_connections primitive already used on membership
removal, for the same reason.

Run the disconnect last and best-effort, after the session record and cookie
are already gone, so sign out completes even when the realtime service is
unreachable.

app/controllers/concerns/authentication.rb

@@ -71,6 +71,13 @@ module Authentication
       Current.session&.destroy!
       reset_session
       remove_authentication_cookie
+      disconnect_remote_connections
+    end
+
+    def disconnect_remote_connections
+      Current.user&.reset_remote_connections
+    rescue => error
+      Rails.logger.warn "Could not disconnect remote connections on sign out: #{error.class}"
     end
 
     def authenticated_as(session)