Separate models into different files

[yld-weng]

Would it be better to separate models by categories:

• models/staff.py
• models/finance.py
• models/client.py
• models/project.py

[yld-weng]

To avoid circular import, use string rather than the model class when creating ForeignKey:

year = models.ForeignKey(FinancialYear, on_delete=models.PROTECT)  

to

year = models.ForeignKey('rse.FinancialYear', on_delete=models.PROTECT)