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.

test/controllers/sessions_controller_test.rb

@@ -56,6 +56,30 @@ class SessionsControllerTest < ActionDispatch::IntegrationTest
     assert_nil Session.find_by(id: session.id)
   end
 
+  test "destroy closes the signed-out user's live connections" do
+    sign_in :david
+
+    remote_connections = mock
+    remote_connections.expects(:disconnect).with(reconnect: true)
+    ActionCable.server.stubs(:remote_connections).returns(mock.tap { |m| m.expects(:where).with(current_user: users(:david)).returns(remote_connections) })
+
+    delete session_url
+
+    assert_redirected_to root_url
+  end
+
+  test "destroy still signs out when the realtime service is unreachable" do
+    sign_in :david
+    session = users(:david).sessions.last
+    ActionCable.server.stubs(:remote_connections).raises(RuntimeError.new("cable down"))
+
+    delete session_url
+
+    assert_redirected_to root_url
+    assert_not cookies[:session_token].present?
+    assert_nil Session.find_by(id: session.id)
+  end
+
   test "destroy removes the push subscription for the device" do
     sign_in :david