[Bug 778118] Add migration to remove document tags

This deletes all tags that point at a document.

migrations/174-remove-tags-from-documents.py

@@ -0,0 +1,16 @@
+from django.contrib.contenttypes.models import ContentType
+
+from taggit.models import TaggedItem
+
+from wiki.models import Document
+
+
+def run():
+    # Get content_type_id for Document.
+    content_type = ContentType.objects.get_for_model(Document)
+    # Get all instances of tags that point at a Document
+    tags = TaggedItem.objects.filter(content_type=content_type)
+
+    print 'Deleting %d tags.' % tags.count()
+    tags.delete()
+    print 'Done!'