bugfix: model migrations

• made CharFields nullable. Currently for backlinks these fields need to be nullable: djangoldp-packages/djangoldp#224
• you should not use on_delete=models.DO_NOTHING.. this makes the database corrupted and will result in IntegrityErrors being thrown later on.. you should use on_delete=models.SET_NULL for most cases or on_delete=models.CASCADE when the changes should be cascaded

Let me know if these changes are okay with the front-end requirements

Cheers! :)