System Hook for Issues

Description

Currently if a user has 100+ projects we'd need to create a project webhook for each one to listen to issue events. It would be easier if there would be a system hook event that we could subscribe for issue changes.

Proposal

Ability to receive events whenever an issue was created/updated/closed/deleted in the entire system.

Links / references

Github has organization webhooks to which you can subscribe for issue events and many more: https://developer.github.com/v3/orgs/hooks/