add postage to notification + noti_history

if it's a letter notification, postage must equal "first" or "second",
else it must equal null

migrations/versions/0228_notification_postage.py

@@ -0,0 +1,23 @@
+"""
+
+Revision ID: 0228_notification_postage
+Revises: 0227_postage_constraints
+Create Date: 2018-09-19 11:42:52.229430
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+revision = '0228_notification_postage'
+down_revision = '0227_postage_constraints'
+
+
+def upgrade():
+    op.add_column('notification_history', sa.Column('postage', sa.String(), nullable=True))
+    op.add_column('notifications', sa.Column('postage', sa.String(), nullable=True))
+
+
+def downgrade():
+    op.drop_column('notifications', 'postage')
+    op.drop_column('notification_history', 'postage')