Add count field for related vulnerabilities

To be able to show an exact count of the related vulnerabilities, we need to expose a count field in the relatedVulnerabilities field of the vulnerabilities work item widget.

Implementation steps

1. Add a new type CountableVulnerabilityType which extends VulnerabilityType and set its connection_type_class to Types::CountableConnectionType
2. Use this CountableVulnerabilityType for the related_vulnerabilities in vulnerabilities_type.rb