Add 'stubbed' column to broadcast_message table

This is a boolean column. It will be set to True for broadcasts
created from training broadcast accounts.

This will help us debug, for example by excluding all the stubbed
broadcasts when we have some trouble with real broadcasts.

migrations/versions/0340_stub_training_broadcasts.py

@@ -0,0 +1,25 @@
+"""
+
+Revision ID: 0340_stub_training_broadcasts
+Revises: 0339_service_billing_details
+Create Date: 2021-01-26 16:48:44.921065
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+revision = '0340_stub_training_broadcasts'
+down_revision = '0339_service_billing_details'
+
+
+def upgrade():
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.add_column('broadcast_message', sa.Column('stubbed', sa.Boolean(), nullable=True))
+    # ### end Alembic commands ###
+
+
+def downgrade():
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_column('broadcast_message', 'stubbed')
+    # ### end Alembic commands ###