Base classes for off study process

The offstudy model is linked to scheduled models by the visit schedule.

# visit_schedule.py
...
visit_schedule1 = VisitSchedule(
    name='visit_schedule1',
    offstudy_model='edc_offstudy.subjectoffstudy',
    ...)
...

This module includes an offstudy model SubjectOffstudy.

You may also declare your own using the OffstudyModelMixin:

class SubjectOffstudy(OffstudyModelMixin, BaseUuidModel):

     pass

If you declare your own, be sure to reference it correctly in the visit schedule:

# visit_schedule.py
...
visit_schedule1 = VisitSchedule(
    name='visit_schedule1',
    offstudy_model='myapp.subjectoffstudy',
    ...)
...

When the offstudy model is saved, the data is validated relative to the consent and visit model. An offstudy datetime should make sense relative to these model instances for the subject. Unused appointments in the future relative to the offstudy datetime will be removed.

Note: There is some redundancy with this model and the offschedule model from edc-visit-schedule. This needs to be resolved.