Enhancement: Registering quick actions as extension

Quick actions can now registered as extension via our extension registry. They need to be of type `action` and have the `files.quick-action` scope.

The old way of registering quick actions via the `quickaction` property of an app is now officially deprecated.

https://github.com/owncloud/web/pull/10102
https://github.com/owncloud/web/issues/7338
