Make it possible to reverse migrations

Conflicts:
	CONTRIBUTORS.rst

CHANGELOG.txt

@@ -19,6 +19,8 @@ Changelog
 1.12.2 (xx.xx.xxxx) - IN DEVELOPMENT
 ~~~~~~~~~~~~~~~~~~~
 
+ * Fix: Migration for addition of ``Page.draft_title`` field is now reversible (Venelin Stoykov)
+
 
 1.12.1 (30.08.2017)
 ~~~~~~~~~~~~~~~~~~~

CONTRIBUTORS.rst

@@ -249,6 +249,7 @@ Contributors
 * Levi Adler
 * Edwar Baron
 * Tomasz Knapik
+* Venelin Stoykov
 * Emily Horsman
 * jcronyn
 * Ben Sturmfels

docs/releases/1.12.2.rst

@@ -13,3 +13,4 @@ What's new
 Bug fixes
 ~~~~~~~~~
 
+ * Migration for addition of ``Page.draft_title`` field is now reversible (Venelin Stoykov)

wagtail/wagtailcore/migrations/0040_page_draft_title.py

@@ -25,5 +25,5 @@ class Migration(migrations.Migration):
             field=models.CharField(default='', editable=False, max_length=255),
             preserve_default=False,
         ),
-        migrations.RunPython(draft_title),
+        migrations.RunPython(draft_title, migrations.RunPython.noop),
     ]