title: How to filter JSON Array in Django JSONField
body: <p>i am getting crazy with filtering a (postgres) JSONField in Django 2.0.3.
  The json is stored as an array. E.g.</p>
  
  <pre><code>tasks = [{"task":"test","level":"10"},{"task":"test 123","level":"20"}]
  </code></pre>
  
  <p>What i've tried:</p>
  
  <pre><code>myModel.objects.filter("tasks__task__contains"="test")
  myModel.objects.filter("tasks_task__0__contains"="test")
  myModel.objects.filter("tasks__0__task__contains"="test")
  myModel.objects.filter("tasks_task__0_x__contains"="test")
  myModel.objects.filter("tasks__0_x__task__contains"="test")
  </code></pre>
  
  <p>What goes wrong?
  What i want to do is a icontains - but as i already read there is not support for icontains on jsonfields in Django right now...</p>