Add custom notifications for webhooks

We want to inform users when there is a problem with their webhooks (especially problems that cause webhooks to be disabled).

To make this possible we could:

• Allow users to register ways to notify them of these problems, including
  • email
  • slack (assumes use of slack integration) - or other chat integration
  • SMS message
  • Create incident/issue with custom template
• Send messages to these channels if the webhook changes state

optionally, allow this to be configured to use a project or group's default notification settings (using cascading settings to propagate values)