Add DB table for service broadcast settings

This will allow us to store details of which channel a service should be
sending to.

See the comment about how all broadcast services can have a row in the
table but may not at the moment. This has been done for speed as it's
the quickest way to let us set up different services to send to
different channels for some needed testing with the mobile handset
providers in the coming week.

migrations/versions/0342_service_broadcast_settings.py

@@ -0,0 +1,43 @@
+"""
+
+Revision ID: 0342_service_broadcast_settings
+Revises: 0341_new_letter_rates
+Create Date: 2021-01-28 21:30:23.102340
+
+"""
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects import postgresql
+
+revision = '0342_service_broadcast_settings'
+down_revision = '0341_new_letter_rates'
+
+CHANNEL_TYPES = ["test", "severe"]
+
+
+def upgrade():
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.create_table('broadcast_channel_types',
+    sa.Column('name', sa.String(length=255), nullable=False),
+    sa.PrimaryKeyConstraint('name')
+    )
+    op.create_table('service_broadcast_settings',
+    sa.Column('service_id', postgresql.UUID(as_uuid=True), nullable=False),
+    sa.Column('channel', sa.String(length=255), nullable=False),
+    sa.Column('created_at', sa.DateTime(), nullable=False),
+    sa.Column('updated_at', sa.DateTime(), nullable=True),
+    sa.ForeignKeyConstraint(['channel'], ['broadcast_channel_types.name'], ),
+    sa.ForeignKeyConstraint(['service_id'], ['services.id'], ),
+    sa.PrimaryKeyConstraint('service_id')
+    )
+    # ### end Alembic commands ###
+
+    for channel in CHANNEL_TYPES:
+      op.execute(f"INSERT INTO broadcast_channel_types VALUES ('{channel}')")
+
+
+def downgrade():
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_table('service_broadcast_settings')
+    op.drop_table('broadcast_channel_types')
+    # ### end Alembic commands ###