Definition of `Tag` model and schema is misleading (and incorrect)

[NamesJ]

Currently, the Tag schema is ('entity_type', 'entity_value', 'tag').
Example data: {'entity_type': 'User.username', 'entity_value: 'ClaustrophobicSkeleton', 'tag': 'baked goods'}.

This works, but entity_type being set as a field for a specific schema seems incorrect and potentially confusing. Ditto for having a tag field in the Tag schema.

A more appropriate schema for Tag would be ('entity_type', 'field_type, 'field_value', 'tag_value').
Example data: {'entity_type': 'User', 'field_type': 'username', 'field_value': 'ClaustrophobicSkeleton', 'tag_value': 'baked goods'}