Notification webhook framework #147

It would be nice if there was a global settings page and the framework to enable activity log notifications.

Notification when a page is created/modified/deleted
Notification when a user is created/modified/deleted
Notification when a file is created/modified/deleted

Notifications could be sent to several possible endpoints, but Slack and Rocket.Chat would be likely good first choices. Eventual additions could be email or messenger platform notifications to the admins