During the migration the automated script to update the copyright
headers accidentally got rid of some of the existing copyright lines.
Reinstate them.
While `ReplicationEndpoint`s register themselves via `JsonResource`,
they pass a method that calls the handler, instead of the handler itself,
to `register_paths`. As a result, `JsonResource` will not correctly pick
up the `@cancellable` flag and we have to apply it ourselves.
Signed-off-by: Sean Quah <seanq@element.io>