Use Issue and Merge Request resource class to determine how to infer the labels

Previously the method #new_stage_and_group_labels_from_intelligent_inference uses infer_from_category: false for merge request label inferrence.

With !390 (comment 281173182), the inference between an Issue and Merge Request are becoming orthogonal.

I'm suggesting to change from using infer_from_category flag to determine whether to infer for an MR or an issue to using the resource class itself.