Add sort by 'Projects' in Vulnerability overview

### Current Behavior:
Currently when viewing vulnerabilities you may sort on `Name` and `Published`.

### Proposed Behavior:
Add sorting based on number of projects affected, ie. the `Projects` column.
If possible then it wouldn't hurt to add sorting based on `Severity` too

Start in the vulnerability overview table and inspect how sorting for the existing Name and Published columns is implemented. Add sorting by the number of affected projects, and consider Severity as a separate enhancement; done when the overview orders rows correctly by the requested fields.

Written by the indexing model from the issue text.