Vue merge request list app backend should support negated assignees

To fully match the non-Vue merge request list, the filtered search bar needs to support - at least - the negated filter for assignees (!=).

The back end for the GraphQL does not currently support this feature, so it needs to be updated.

Additionally, the issues list supports "One of these" for assignees (||). Neither the existing merge request list nor the new Vue list currently supports this filter option.