GraphQL: Add 'label_priority' sort for Issuables

Add GraphQL sorting for label_priority on an Issuable

        when 'label_priority', 'label_priority_asc'           then order_labels_priority(excluded_labels: excluded_labels)
        when 'label_priority_desc'                            then order_labels_priority('DESC', excluded_labels: excluded_labels)

in https://gitlab.com/gitlab-org/gitlab/blob/master/app/models/concerns/issuable.rb#L230-247

Will require updating Types::IssuableSortEnum

Due to the complexity of the sort, may require updates to the keyset pagination support