Migration for new letter pricing

We now support letters of up to 5 sheets long, so we need to store the
rates for 4 and 5 sheet letters (both crown and non-crown) in the
`letter_rates` table.

migrations/versions/0202_new_letter_pricing.py

@@ -0,0 +1,37 @@
+"""empty message
+
+Revision ID: 0202_new_letter_pricing
+Revises: 0201_another_letter_org
+Create Date: 2017-07-09 12:44:16.815039
+
+"""
+
+revision = '0202_new_letter_pricing'
+down_revision = '0201_another_letter_org'
+
+import uuid
+from datetime import datetime
+from alembic import op
+
+
+start = datetime(2018, 6, 30, 23, 0)
+
+NEW_RATES = [
+    (uuid.uuid4(), start, 4, 0.39, True, 'second'),
+    (uuid.uuid4(), start, 4, 0.51, False, 'second'),
+    (uuid.uuid4(), start, 5, 0.42, True, 'second'),
+    (uuid.uuid4(), start, 5, 0.57, False, 'second'),
+]
+
+
+def upgrade():
+    conn = op.get_bind()
+    for id, start_date, sheet_count, rate, crown, post_class in NEW_RATES:
+        conn.execute("""
+            INSERT INTO letter_rates (id, start_date, sheet_count, rate, crown, post_class)
+                VALUES ('{}', '{}', '{}', '{}', '{}', '{}')
+        """.format(id, start_date, sheet_count, rate, crown, post_class))
+
+
+def downgrade():
+    pass