adding TombstoneMixin to behavior-related api serializers
adding TombstoneMixin to behavior-related api serializers

izzyalonso/tndata_backend,izzyalonso/tndata_backend,tndatacommons/tndata_backend,izzyalonso/tndata_backend,tndatacommons/tndata_backend,izzyalonso/tndata_backend,tndatacommons/tndata_backend,tndatacommons/tndata_backend