Add option to create folders in the project and group projects

### Current Behavior

There is only option to create a project of existing type and assign projects under it.

### Proposed Behavior

Add option to create plain folder, without any Identity tab (Namespace, Package URL, etc.).

When permissions are assigned into this folder, users should have access to all children.

Under vulnerabilities, there should be total number of them for all the children. Detail of this folder should display same things as the project index, that means portfolio vulnerabilities, projects at risk etc., but only for children in that folder.

It will make it easier to manage more projects, easily assign permissions to corresponding teams.

### 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