ForeignKey on_delete missing

[jayvdb]

  File "/usr/lib/python3.8/site-packages/command_scheduler/models.py", line 66, in Log
    command = models.ForeignKey(Command, related_name='logs')
TypeError: __init__() missing 1 required positional argument: 'on_delete'

Usually an easy fix. I guess the appropriate thing to do is not delete logs when a command is deleted.

Deleting logs with the command might be useful, but should be opt-in.