Add sorting to security dashboard column: Severity

See Design Issue and parent Epics for details.

Additional information

The severity column needs a sorting. See the design here.

Implementation Plan

• Add sorting properties to the gl-table in the ee/app/assets/javascripts/security_dashboard/components/vulnerability_list.vue file. An example of how to do it can be found below.

Additional context

The app/assets/javascripts/alert_management/components/alert_management_table.vue file has an example implementation on how to sort a column. Fortunately the gl-table has support for sorting so we just need to implement the sorting function.