Make it possible to unfollow someone

I was sure it was already implemented, but it looks like it wasn't

migrations/2018-09-04-103017_follows_add_ap_url/down.sql

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

migrations/2018-09-04-103017_follows_add_ap_url/up.sql

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+-- Your SQL goes here
+ALTER TABLE follows ADD COLUMN ap_url TEXT NOT NULL DEFAULT '';