Unapprove reviewers hook should be an event listener

Forks have thrown a bit of a spanner in the works of the Unapprove Reviewers Hook doesn't make a lot of sense. Enabling it for a repository does not mean that updates to cross-repository pull requests will necessarily unapprove reviewers - it would need to be enabled on both the source & target repository for it to be guaranteed to work.

I'm thinking this plugin should no longer be a hook. Instead it might be best re-implemented as an event listener (listening for the PullRequestRescopedEvent - https://developer.atlassian.com/static/javadoc/stash/2.5.0/api/reference/com/atlassian/stash/event/pull/PullRequestRescopedEvent.html) with a checkbox that can be toggled on and off per repository in the Pull Request settings page, rather than as a repository hook.