Add support for avatars, and fetch remote ones

migrations/2018-09-03-102510_users_add_avatar/down.sql

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

migrations/2018-09-03-102510_users_add_avatar/up.sql

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+-- Your SQL goes here
+ALTER TABLE users ADD COLUMN avatar_id INTEGER REFERENCES medias(id) ON DELETE CASCADE;