Next phase Notifications evolution

### Problem description
#2426 introduced cross-user (workflow) notifications into Guardian. At present the configuration of notifications, their filtering and frequency are hard-coded. More flexibility is required.

### Requirements
Introduce the ability for users to configure which notifications they want to receive and how:

- a choice, per subject, of no notification at all, all notifications, throttling notifications for after certain number within the certain time period
- works with multi-policy chains within a single instance of Guardian, as well as Modules and Tools
- ability to trigger the notification from within the Policy
- ability to attach a message to the Notification when triggered explicitly
- a packaged-in (out-of-the box) set of sensible notifications which covers the majority of deployment scenarious

### Definition of done

- Functionality is implemented as per the requirements section above
- Documentation is created
- Example notifications are implemented in the packaged policies

### Acceptance criteria
Users can configure the display of notifications via Guardian UI