The intention of issue #8115 seems to be to consolidate unsubscribe
behaviour. This commit attempts to formalize this by handling all
unsubscribe routes with the same exact handler.
It changes the behaviour of unsubscribe-notifications to be consistent
as discussed in pull request #8135.
One file that is not tracked here is rev-manifest.json.
It is generated at runtime and its contents changes as the contents
of client side files change.