[1.5.x] Fixed #19441 -- Created PostgreSQL varchar index when unique=True

Thanks Dylan Verheul for the report and Anssi Kääriäinen for the
review.
Backport of 55972ee5c from master.

tests/regressiontests/indexes/models.py

@@ -9,3 +9,9 @@ class Article(models.Model):
         index_together = [
             ["headline", "pub_date"],
         ]
+
+
+class IndexedArticle(models.Model):
+    headline = models.CharField(max_length=100, db_index=True)
+    body = models.TextField(db_index=True)
+    slug = models.CharField(max_length=40, unique=True, db_index=True)