title: django postgresql json field schema validation
body: <p>I have a django model with a <code>JSONField</code> (django.contrib.postgres.fields.JSONField)
  Is there any way that I can validate model data against a json schema file? </p>
  
  <p>(pre-save)<br>
  Something like <code>my_field = JSONField(schema_file=my_schema_file)</code></p>