separate notification postage constraint into three separate commits

we had an issue where the notification postage constraint command ran
into a deadlock, after trying to acquire two exclusive access locks on
large frequently modified/read tables.

To avoid this happening, we've had to split the upgrade script into
three - one script to apply the not-valid constraint to notifications
table, one for notification_history, and a third to validate the two
constraints.

Note: The first two scripts acquire exclusive access locks, but the
third only needs a row by row lock.

since this involves changing the exsiting alembic upgrades, if you've
upgraded your db you'll need to run the following three commands to
revert your database to a previous good state.

```
alter table notifications drop constraint chk_notifications_postage_null;
alter table notification_history drop constraint chk_notification_history_postage_null;
update alembic_version set version_num = '0229_new_letter_rates';
```

migrations/versions/0230_noti_postage_constraint.py

@@ -1,44 +0,0 @@
-"""
-
-Revision ID: 0230_noti_postage_constraint
-Revises: 0229_new_letter_rates
-Create Date: 2018-09-19 11:42:52.229430
-
-"""
-from alembic import op
-
-
-revision = '0230_noti_postage_constraint'
-down_revision = '0229_new_letter_rates'
-
-
-def upgrade():
-    op.execute("""
-        ALTER TABLE notifications ADD CONSTRAINT "chk_notifications_postage_null"
-        CHECK (
-            CASE WHEN notification_type = 'letter' THEN
-                postage is not null and postage in ('first', 'second')
-            ELSE
-                postage is null
-            END
-        )
-        NOT VALID
-    """)
-    op.execute("""
-        ALTER TABLE notification_history ADD CONSTRAINT "chk_notification_history_postage_null"
-        CHECK (
-            CASE WHEN notification_type = 'letter' THEN
-                postage is not null and postage in ('first', 'second')
-            ELSE
-                postage is null
-            END
-        )
-        NOT VALID
-    """)
-    op.execute('ALTER TABLE notifications VALIDATE CONSTRAINT "chk_notifications_postage_null"')
-    op.execute('ALTER TABLE notification_history VALIDATE CONSTRAINT "chk_notification_history_postage_null"')
-
-
-def downgrade():
-    op.drop_constraint('chk_notifications_postage_null', 'notifications', type_='check')
-    op.drop_constraint('chk_notification_history_postage_null', 'notification_history', type_='check')

migrations/versions/0230_noti_postage_constraint_1.py

@@ -0,0 +1,30 @@
+"""
+
+Revision ID: 0230_noti_postage_constraint_1
+Revises: 0229_new_letter_rates
+Create Date: 2018-09-19 11:42:52.229430
+
+"""
+from alembic import op
+
+
+revision = '0230_noti_postage_constraint_1'
+down_revision = '0229_new_letter_rates'
+
+
+def upgrade():
+    op.execute("""
+        ALTER TABLE notifications ADD CONSTRAINT "chk_notifications_postage_null"
+        CHECK (
+            CASE WHEN notification_type = 'letter' THEN
+                postage is not null and postage in ('first', 'second')
+            ELSE
+                postage is null
+            END
+        )
+        NOT VALID
+    """)
+
+
+def downgrade():
+    op.drop_constraint('chk_notifications_postage_null', 'notifications', type_='check')

migrations/versions/0231_noti_postage_constraint_2.py

@@ -0,0 +1,30 @@
+"""
+
+Revision ID: 0230_noti_postage_constraint_2
+Revises: 0230_noti_postage_constraint_1
+Create Date: 2018-09-19 11:42:52.229430
+
+"""
+from alembic import op
+
+
+revision = '0230_noti_postage_constraint_2'
+down_revision = '0230_noti_postage_constraint_1'
+
+
+def upgrade():
+    op.execute("""
+        ALTER TABLE notification_history ADD CONSTRAINT "chk_notification_history_postage_null"
+        CHECK (
+            CASE WHEN notification_type = 'letter' THEN
+                postage is not null and postage in ('first', 'second')
+            ELSE
+                postage is null
+            END
+        )
+        NOT VALID
+    """)
+
+
+def downgrade():
+    op.drop_constraint('chk_notification_history_postage_null', 'notification_history', type_='check')

migrations/versions/0232_noti_postage_constraint_3.py

@@ -0,0 +1,21 @@
+"""
+
+Revision ID: 0230_noti_postage_constraint_3
+Revises: 0230_noti_postage_constraint_2
+Create Date: 2018-09-19 11:42:52.229430
+
+"""
+from alembic import op
+
+
+revision = '0230_noti_postage_constraint_3'
+down_revision = '0230_noti_postage_constraint_2'
+
+
+def upgrade():
+    op.execute('ALTER TABLE notifications VALIDATE CONSTRAINT "chk_notifications_postage_null"')
+    op.execute('ALTER TABLE notification_history VALIDATE CONSTRAINT "chk_notification_history_postage_null"')
+
+
+def downgrade():
+    pass