Django 2.0 support

[mrabedini]

while using with django 2.0 I get the following error:

 File "../stored_messages/models.py", line 34, in MessageArchive
    user = models.ForeignKey(settings.AUTH_USER_MODEL)
TypeError: __init__() missing 1 required positional argument: 'on_delete'

It seems that in django2.0 the on_delete property is not mandatory. It should explicitly be expressed.