Consider using a proper multi-select component for filters in Group Security Dashboard

The Group Security Dashboard currently uses gl-dropdown components to implement its filters. Because multiple items within each dropdown can be toggled on at the same time, it could be argued that it'd be better implemented via a dedicated select[multiple]-like component, e.g., vue-multiselect.

As it is, an inelegant workaround is required to prevent the default behaviour of the gl-dropdown: closing when any item within it is clicked.

This possibly conflicts with #10530.