Supersedes https://github.com/element-hq/synapse/pull/17958.
Awkwardly, the changes made to fix the mypy errors in 1.12.1 cause
errors in 1.11.2. So you'll need to update your mypy version to 1.12.1
to eliminate typechecking errors during developing.
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>
