Implement sort by Affected Projects in Vulnerabilities list

### Current Behavior

Currently the list of all vulnerabilities cannot be sorted by affected project count.

### Proposed Behavior

Allow sorting the list of vulnerabilities by affected project count, as requested in https://github.com/DependencyTrack/frontend/issues/122

There can be several reasons to work on the most occurring vulnerabilities:
- triage them to make the state of the portfolio more accurate / up-to-date
- upgrade the affected component across the whole portfolio to reduce the risk of the portfolio
- ...

For us it's not uncommon to work on a specific vulnerability and have someone fix / upgrade the component in all the projects in the portfolio. Being able to sort on affected projects count will help to work on the most common vulnerabilities.

### 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