Update first class letter rate start dates

We want to bring the start dates for first class letter rates forward by
a month so that we don't see billing errors when sending first class letters now.
(The feature will still go live at the planned time - this is to let us test things
beforehand.)

migrations/versions/0233_updated_first_class_dates.py

@@ -0,0 +1,30 @@
+"""empty message
+
+Revision ID: 0233_updated_first_class_dates
+Revises: 0230_noti_postage_constraint_3
+
+"""
+
+revision = '0233_updated_first_class_dates'
+down_revision = '0230_noti_postage_constraint_3'
+
+from datetime import datetime
+from alembic import op
+from sqlalchemy.sql import text
+
+
+START_DATE = datetime(2018, 8, 31, 23, 0)
+
+
+def upgrade():
+    conn = op.get_bind()
+    conn.execute(text(
+        """UPDATE letter_rates SET start_date = :start_date WHERE post_class = 'first'"""
+    ), start_date=START_DATE)
+
+
+def downgrade():
+    '''
+    This data migration should not be downgraded. Downgrading may cause billing errors
+    and the /montly-usage endpoint to stop working.
+    '''