Create FarmActivity (or FarmLog) model

[arruda]

Should have a mandatory start-datetime, optional timedelta duration field, and description. In farmOS the base class is FarmLog, but FarmActivity seems like something that could be the Parent of any activity that is logged into the system. Indeed FarmActivity is nothing more than a FarmLog with an different name in FarmOS(it ads no new fields or logic).

Also should have status choice field (pending, done)

Inherity or ref and occurrence from https://django-scheduler.readthedocs.io/

[arruda]

not using django-scheduler, old, unmaintained and too complicated for what we actually want