title: Django UniqueConstraint case insensitive and unaccented
body: <p>I know I could get unique case insensitive field by using
  <code>from django.contrib.postgres.fields import CICharField</code>, but I couldn't get it work by something like</p>
  <pre class="lang-py prettyprint-override"><code>    class Meta:
          constraints = [
              models.UniqueConstraint(
                  fields=[&quot;name__unaccent&quot;],
                  name=&quot;unique name&quot;,
              ),
          ]
  </code></pre>
  <p>I have <code>UnaccentExtension</code>, but this still throws error</p>
  <pre><code>django.core.exceptions.FieldDoesNotExist: Group has no field named 'name__unaccent'
  </code></pre>
  <p>Am I doing something wrong? Is it somehow possible achieve, what I am looking for?</p>