Fixed #26709 -- Added class-based indexes.

Added the AddIndex and RemoveIndex operations to use them in migrations.

Thanks markush, mjtamlyn, timgraham, and charettes for review and advice.

tests/model_indexes/models.py

@@ -0,0 +1,7 @@
+from django.db import models
+
+
+class Book(models.Model):
+    title = models.CharField(max_length=50)
+    author = models.CharField(max_length=50)
+    pages = models.IntegerField(db_column='page_count')