Non-generic relations

Add abstract models to enable users to implement non-generic relations between entities, attributes, and values.

## Detailed Description
I would like to define my own relations between entities (of known types) and attributes and values, rather than using Django's contenttypes framework and generic relations.

## Context
If this library offers abstract models (in addition to, and as an alternative to, the concrete models that define generic relations), users could benefit from this library while (arguably) enjoying better customizability and performance.

## Possible Implementation
I'm working on a draft PR.

Start by reviewing the concrete models that currently define generic relations and the library's existing entity, attribute, and value relationships. Compare those structures with the draft PR mentioned in the issue. Done means users can implement non-generic relations through abstract models as an alternative to the existing concrete models.

Written by the indexing model from the issue text.