Create DailySortedLetter table

The response files we receive from the DVLA when we send letters
contain a row for each letter and a field with a value of 'Unsorted' or
'Sorted'. This table will be used to store the total number of
'Unsorted' and 'Sorted' letter notifications per day.

migrations/versions/0173_create_daily_sorted_letter.py

@@ -0,0 +1,30 @@
+"""
+
+Revision ID: 0173_create_daily_sorted_letter
+Revises: 0172_deprioritise_examples
+Create Date: 2018-03-01 11:53:32.964256
+
+"""
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects import postgresql
+
+revision = '0173_create_daily_sorted_letter'
+down_revision = '0172_deprioritise_examples'
+
+
+def upgrade():
+    op.create_table('daily_sorted_letter',
+    sa.Column('id', postgresql.UUID(as_uuid=True), nullable=False),
+    sa.Column('billing_day', sa.Date(), nullable=False),
+    sa.Column('unsorted_count', sa.Integer(), nullable=False),
+    sa.Column('sorted_count', sa.Integer(), nullable=False),
+    sa.Column('updated_at', sa.DateTime(), nullable=True),
+    sa.PrimaryKeyConstraint('id')
+    )
+    op.create_index(op.f('ix_daily_sorted_letter_billing_day'), 'daily_sorted_letter', ['billing_day'], unique=True)
+
+
+def downgrade():
+    op.drop_index(op.f('ix_daily_sorted_letter_billing_day'), table_name='daily_sorted_letter')
+    op.drop_table('daily_sorted_letter')