Add a type table for broadcast providers

This adds a type table for broadcast providers, which is the pattern we
follow with our models (e.g. we have a `broadcast_channel_types` table).

As well as the four providers, the migration populates it with `all`
which is the value that will replace `null` in a later change.

It should be safe to add the foreign key constraint to the
`service_broadcast_settings` in the same migration since the column is
still nullable and we don't have data in that column that is not in the
types table.

migrations/versions/0352_broadcast_provider_types.py

@@ -0,0 +1,32 @@
+"""
+
+Revision ID: 0352_broadcast_provider_types
+Revises: 0351_unique_key_annual_billing
+Create Date: 2021-05-05 15:07:22.146657
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+revision = '0352_broadcast_provider_types'
+down_revision = '0351_unique_key_annual_billing'
+
+PROVIDER_TYPES = ('ee', 'three', 'vodafone', 'o2', 'all')
+
+
+def upgrade():
+    op.create_table('broadcast_provider_types',
+                    sa.Column('name', sa.String(length=255), nullable=False),
+                    sa.PrimaryKeyConstraint('name'))
+    for provider in PROVIDER_TYPES:
+        op.execute(f"INSERT INTO broadcast_provider_types VALUES ('{provider}')")
+    op.create_foreign_key('service_broadcast_settings_provider_fkey',
+                          'service_broadcast_settings',
+                          'broadcast_provider_types',
+                          ['provider'],
+                          ['name'])
+
+
+def downgrade():
+    op.drop_constraint('service_broadcast_settings_provider_fkey', 'service_broadcast_settings', type_='foreignkey')
+    op.drop_table('broadcast_provider_types')