Add operator channel migration

Looks identical to the government channel migration 354

migrations/versions/0358_operator_channel.py

@@ -0,0 +1,22 @@
+"""
+
+Revision ID: 0358_operator_channel
+Revises: 0357_validate_constraint
+Create Date: 2021-06-09 13:44:12.479191
+
+"""
+from alembic import op
+
+revision = '0358_operator_channel'
+down_revision = '0357_validate_constraint'
+
+
+def upgrade():
+    op.execute("INSERT INTO broadcast_channel_types VALUES ('operator')")
+
+
+def downgrade():
+    # This can't be downgraded if there are rows in service_broadcast_settings which
+    # have the channel set to operator or if broadcasts have already been sent on the
+    # operator channel - it would break foreign key constraints.
+    op.execute("DELETE FROM broadcast_channel_types WHERE name = 'operator'")