Add a cover field to posts

Referencing the media to use to illustrate the article

migrations/postgres/2018-10-30-151529_add_post_cover/down.sql

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+-- This file should undo anything in `up.sql`
+ALTER TABLE posts DROP COLUMN cover_id;

migrations/postgres/2018-10-30-151529_add_post_cover/up.sql

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+-- Your SQL goes here
+ALTER TABLE posts ADD COLUMN cover_id INTEGER REFERENCES medias(id) DEFAULT NULL;