Migration to add international letter rates

New rows giving the prices of letters with a post_class of `europe` and
`rest-of-world` have been added to the `letter_rates` table. All rates
are currently the same for international letters.

migrations/versions/0324_int_letter_rates.py

@@ -0,0 +1,54 @@
+"""
+
+Revision ID: 0324_int_letter_rates
+Revises: 0323_broadcast_message
+Create Date: 2020-07-08 12:20:20.700128
+
+"""
+import itertools
+import uuid
+from datetime import datetime
+
+from alembic import op
+from sqlalchemy.sql import text
+
+from app.models import LetterRate
+
+
+revision = '0324_int_letter_rates'
+down_revision = '0323_broadcast_message'
+
+base_rate = 76
+start_date = datetime(2020, 7, 1, 0, 0)
+
+
+def upgrade():
+    """
+    Insert these letter rates for a post_class of both `europe` and `rest-of-world`:
+    1 sheet - £0.84
+    2 sheets - £0.92
+    3 sheets - £1.00
+    4 sheets - £1.08
+    5 sheets - £1.16
+    """
+    op.bulk_insert(LetterRate.__table__, [
+        {
+            'id': uuid.uuid4(),
+            'start_date': start_date,
+            'end_date': None,
+            'sheet_count': sheet_count,
+            'rate': (base_rate + (8 * sheet_count)) / 100.0,
+            'crown': crown,
+            'post_class': post_class,
+        }
+        for sheet_count, crown, post_class in itertools.product(
+            range(1, 6),
+            [True, False],
+            ['europe', 'rest-of-world']
+        )
+    ])
+
+
+def downgrade():
+    conn = op.get_bind()
+    conn.execute(text("DELETE FROM letter_rates WHERE start_date = :start"), start=start_date)