Refactor Domain Item and Domain Entity Types

[michaelsframe]

Domain Items and Domain Entity Types should be chosen by the user. To do so we need to modify the Domain Item and Entity Type models.

This includes the method to add an item and type from an Extracted Entity in the Depot. The first step is to modify the models. Second step is to modify the Depot extracted entity page so that the user can click an Extracted Item to add it or its Type to a Domain. Third step will be a separate issue to enable this functionality from the Extracted Entity Panel Widget #43 .