Add postage column to ft_billing

Added a new varchar column, postage, to ft_billing. This is nullable and
not part of the composite primary key for now, but this will change
later.

migrations/versions/0234_ft_billing_postage.py

@@ -0,0 +1,22 @@
+"""
+
+Revision ID: 0234_ft_billing_postage
+Revises: 0233_updated_first_class_dates
+Create Date: 2018-09-28 14:43:26.100884
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+revision = '0234_ft_billing_postage'
+down_revision = '0233_updated_first_class_dates'
+
+
+def upgrade():
+    op.add_column('ft_billing', sa.Column('postage', sa.String(), nullable=True))
+    op.execute("UPDATE ft_billing SET postage = (CASE WHEN notification_type = 'letter' THEN 'second' ELSE 'none' END)")
+
+
+def downgrade():
+    op.drop_column('ft_billing', 'postage')