"Fix" various model issues

* handle `property` attributes in `Builder.publish_link`
* filter out `None` in `Builder.publish_link_collection`
* add `Documentable` and `Personable` to several models:
  Cycle, Directive, Program, System
* add `relationship_type` to RelationshipType model to match previous
  schema
* make `ChangeTracked.modified_by_id` nullable, pending Account model
  and migration
* fix typo in `Relationship.get_relationship_node`