Generalized database schema

Here is a generalized DB schema that takes advantage of registries in `identifiers.org`. I added some comments to explain the purpose.

```python
from django.db import models

# Registry in identifiers.org
class Registry(models.Model):
name = models.CharField()
prefix = models.CharField()
description = models.CharField()
# and other attributes of a registry ...

# Entity includes common attributes of any entity (such as gene,
# publication, disease, tissue, etc)
class Entity(models.Model):
accession = models.CharField(null=True) # accession in identifiers.org
registry = models.ForeignKey(Registry, null=True)
# and other attributes shared by all entities ...

# "Gene" is one kind of entity
class Gene(models.Model):
entity = models.OneToOneField(
Entity,
on_delete=models.CASCADE,
primary_key=True,
)
# specific attributes for a gene
scientific_name = models.CharField(max_length=32)
systematic_name = models.CharField(max_length=32)
organism = models.ForeignKey(Organism, ...)

# "Publication" is another kind of entity
class Publication(models.Model):
entity = models.OneToOneField(
Entity,
on_delete=models.CASCADE,
primary_key=True,
)
# specific attributes for a publication
pmid = models.IntegerField(null=True, unique=True, db_index=True)
title = models.TextField()
authors = models.TextField()
date = models.DateField()
journal = models.TextField()
volume = models.TextField(blank=True, null=True)
pages = models.TextField(blank=True, null=True)
issue = models.TextField(blank=True, null=True)

# "Disease" is another kind of entity
class Disease(models.Model):
entity = models.OneToOneField(
Entity,
on_delete=models.CASCADE,
primary_key=True,
)
# and specific attributes for a disease ...

# "Entityset" includes common attributes for any kind of entity set.
# It may include different types of entities.
class Entityset(models.Model):
creator = models.ForeignKey(User)
title = models.TextField()
abstract = models.TextField(null=True)
slug = models.SlugField(help_text="Slugified title field", max_length=75)
public = models.BooleanField(default=False)
deleted = models.BooleanField(default=False)
fork_of = models.ForeignKey('self', editable=False, null=True)
tip_item_count = models.IntegerField(null=True)

# "Geneset" is one kind of "Entityset"
class Geneset(models.Model):
entityset = models.OneToOneField(
Entityset,
on_delete=models.CASCADE,
primary_key=True,
)
organism = models.ForeignKey(Organism)
# and other attributes for a geneset

# Similar models can be defined for "Publicationset" or "Diseaseset" ...

# Version of an Entityset
class Version(models.Model):
entityset = models.ForeignKey(entityset)
creator = models.ForeignKey(User)
ver_hash = models.CharField(db_index=True, max_length=40)
description = models.TextField(null=True)
commit_date = models.DateTimeField(auto_now_add=True)
parent = models.ForeignKey('self', null=True)

# Annotations of entities
class Annotation(models.Model):
version = models.ForeignKey(Version)
primary_entity = models.ForeignKey(Entity) # entity that is being annotated
annotator_entity = models.ForeignKey(Entity) # entity that is the annotation
```

No repository files or tests are named. Start by reviewing the proposed Django models and the identifiers.org registry relationships; completion would require an agreed schema design and an explicitly defined implementation scope.

Written by the indexing model from the issue text.