Changelog entry (issue #500)

Signed-off-by: Michal Čihař <michal@cihar.com>
Register hook backends using decorator

This allows to simplify the code without explicitly listing all hook
backends in the handler.

Also remove not used csrf decorator on the hooks as they are never
called directly from the view.

See also issue #500.

Signed-off-by: Michal Čihař <michal@cihar.com>