Add github webhook to signal "auto-close" permission

[stephen-soltesz]

We don't want to close an issue before the alert is 'resolved' -- and we don't want the alertmanager-github-receiver to automatically close issues that we haven't yet updated with an explanation. As a compromise, we could support a directive via a github-webhook to signal the alertmanager-github-receiver that it is okay to auto-close an issue. For example, when adding an explanation to the issue, we could at the same time include the directive:

/autoclose

That would result in alertmanager-github-receiver closing the issue once it received an update from Alertmanager.