Support parent-child-relationships for components

### Current Behavior

With 4.7, support for parent-child-relationships of projects was added (#84).

In addition to projects, we want to support the same kind of relationships for components as well.

Currently, component hierarchies as defined by the BOMs uploaded to DT are flattened

### Proposed Behavior

* Maintain component hierarchies when ingesting BOMs
* Add UI metaphors similar to those currently used for projects to display those hierarchies

### 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