Customize notification settings instance-wide

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Zendesk ticket (internal link): https://gitlab.zendesk.com/agent/tickets/92330

Currently, notifications can be customized on 3 levels, with each following level taking precedence over the previous:

• Global (actually the settings for a single user)
• Group
• Project

A customer would like to see a fourth: the instance-wide level. At this instance-wide level, an Admin should be able to customize what kinds of notification emails are allowed on this instance. For example, the Admin can specify that emails that notify a user he/she was added to a group isn't sent at any time on this instance.