* Introduce RelatedObjectCountField
* Introduce get_annotations_for_serializer() and enable dynamic annotations
* Add RelatedObjectCountFields to serializers; remove static annotations from querysets
* Remove annotations cleanup logic from BriefModeMixin
* Annotate type for RelatedObjectCountField
* Remove redundant field on TagSerializer
* Add missing reverse relationship for power feeds to rack
* Refactor RelatedObjectCountField to take a single relationship name