Update created_at column of ft_billing to be non-nullable

`created_at` was added previously and made nullable temporarily. This
commit now populates the column, ensures that it will always have a
value, and makes `created_at` non-nullable.

migrations/versions/0194_ft_billing_created_at.py

@@ -0,0 +1,23 @@
+"""
+
+Revision ID: 0194_ft_billing_created_at
+Revises: 0193_add_ft_billing_timestamps
+Create Date: 2018-05-22 14:34:27.852096
+
+"""
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects import postgresql
+
+revision = '0194_ft_billing_created_at'
+down_revision = '0193_add_ft_billing_timestamps'
+
+
+def upgrade():
+    op.execute("UPDATE ft_billing SET created_at = NOW()")
+    op.alter_column('ft_billing', 'created_at', nullable=False)
+
+
+def downgrade():
+    op.alter_column('ft_billing', 'created_at', nullable=True)
+    op.execute("UPDATE ft_billing SET created_at = null")