Purge broadcast data

To avoid problems after changing data type for areas column from
array to dictionary.

migrations/versions/0329_purge_broadcast_data.py

@@ -0,0 +1,24 @@
+"""
+
+Revision ID: 0329_purge_broadcast_data
+Revises: 0328_international_letters_perm
+Create Date: 2020-09-07 16:00:27.545673
+
+"""
+from alembic import op
+
+
+revision = '0329_purge_broadcast_data'
+down_revision = '0328_international_letters_perm'
+
+
+def upgrade():
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.execute("TRUNCATE broadcast_event, broadcast_message;")
+    # ### end Alembic commands ###
+
+
+def downgrade():
+    # ### commands auto generated by Alembic - please adjust! ###
+    pass
+    # ### end Alembic commands ###