Ability to select policies for notification

### Current Behavior

Currently notifications go out for all policies and all projects in violation.

### Proposed Behavior

It would be very useful to be able to create notifications on selected policies only.

### Checklist

- [X] I have read and understand the [contributing guidelines](https://github.com/DependencyTrack/dependency-track/blob/master/CONTRIBUTING.md#filing-issues)
- [X] I have checked the [existing issues](https://github.com/DependencyTrack/frontend/issues) for whether this enhancement was already requested

No files, tests, or entry points are named. Start by tracing how the frontend represents notification settings and policies, then confirm the intended selection behavior with maintainers; done means notifications can be limited to explicitly selected policies without changing existing all-policy behavior.

Written by the indexing model from the issue text.