New table returned-letters

The table will contain notification ids for services that have returned letters. This will make it easy to query the data in Notification_history since we can join on the primary key.

migrations/versions/0310_returned_letters_table_.py

@@ -0,0 +1,30 @@
+"""
+
+Revision ID: 0310_returned_letters_table
+Revises: 0309_add_uq_key_row_number
+Create Date: 2019-12-09 12:13:49.432993
+
+"""
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects import postgresql
+
+revision = '0310_returned_letters_table'
+down_revision = '0309_add_uq_key_row_number'
+
+
+def upgrade():
+    op.create_table('returned_letters',
+        sa.Column('id', postgresql.UUID(as_uuid=True), nullable=False),
+        sa.Column('reported_at', sa.DateTime(), nullable=False),
+        sa.Column('service_id', postgresql.UUID(as_uuid=True), nullable=False),
+        sa.Column('notification_id', postgresql.UUID(as_uuid=True), nullable=False),
+        sa.ForeignKeyConstraint(['service_id'], ['services.id'], ),
+        sa.PrimaryKeyConstraint('id'),
+        sa.UniqueConstraint('notification_id')
+    )
+    op.create_index(op.f('ix_returned_letters_service_id'), 'returned_letters', ['service_id'], unique=False)
+
+
+def downgrade():
+    op.drop_table('returned_letters')