add custom_domain (default to NULL) to schema

migrations/postgres/2019-05-22-124143_add_custom_domains/down.sql

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+-- undo the adding of custom_domain column to blogs table.
+ALTER TABLE blogs DROP COLUMN custom_domain;

migrations/postgres/2019-05-22-124143_add_custom_domains/up.sql

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+--- Adding custom domain to Blog as an optional field
+ALTER TABLE blogs ADD COLUMN custom_domain VARCHAR DEFAULT NULL UNIQUE;