Fixed issue with sortables.js where SortableGenericTabularInline and
SortableGenericStackedInline would break whenever adding blank inline
forms. This is because inline fieldset names append the names of the
generic relation fields as defined in models.py. For instance, instead
of being called "model_set-n-field" they'd be called
"app-model-content_type-object_id-n-field" reflecting the names of the
generic type and id fields. I replaced the string split on '-' with a
regexp split that looks for the -n- pattern instead. This appears to
work on Chrome for both regular and generic relations.