title: Django full text search: says unaccent does not exist
body: <p>I am using Django 2.0 and postgresql 9.6</p>
  
  <p>I have created and empty migration file in my app (articles) with the following content to add unaccent and trigram extensions</p>
  
  <pre><code># Generated by Django 2.0 on 2018-02-06 22:34
  
  from django.db import migrations
  from django.contrib.postgres.operations import UnaccentExtension, TrigramExtension
  
  
  class Migration(migrations.Migration):
  
      dependencies = [
          ('articles', '0012_auto_20180205_2234'),
      ]
  
      operations = [
          UnaccentExtension(),
          TrigramExtension()
      ]
  </code></pre>
  
  <p>Then i try to run the following query:</p>
  
  <pre><code>from django.contrib.postgres.search import SearchQuery, SearchRank, SearchVector
  vector = SearchVector('title',config='unaccent', weight='A') + SearchVector('description',config='unaccent', weight='B')
  query = SearchQuery('india')
  Article.objects.annotate(rank=SearchRank(vector, query)).filter(rank__gte=0.3).order_by('rank')
  
  ProgrammingError: text search configuration "unaccent" does not exist
  LINE 1: ...rticle"."qa_bool", ts_rank((setweight(to_tsvector('unaccent'...
  </code></pre>