Limit notifications to portfolio(s) instead of/alongside projects

### Current Behavior

Currently notifications can only be limited to projects. This means that any new project has to be added in the notification manually (unless no limits are set of course). Since this can only be done by a user with administrative permissions, this is a rather involved process and takes more time than it really should.

### Proposed Behavior

It would be good to be able to limit notifications to a portfolio, so that any new projects in the portfolio are automatically picked up.
Seeing that (at least for the mail notification) it is already possible to send it to a specific team, maybe there it could even be the default -- seeing how a team should only see their own portfolio.

### 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/dependency-track/issues) for whether this enhancement was already requested