Allow for media cover deletion (#387)

* Allow for media cover deletion

Fix #356

* Fix sqlite migrations

migrations/postgres/2018-12-25-164502_media-cover-deletion/down.sql

@@ -0,0 +1,4 @@
+-- This file should undo anything in `up.sql`
+
+ALTER TABLE posts DROP CONSTRAINT posts_cover_id_fkey;
+ALTER TABLE posts ADD CONSTRAINT posts_cover_id_fkey FOREIGN KEY (cover_id) REFERENCES medias(id);

migrations/postgres/2018-12-25-164502_media-cover-deletion/up.sql

@@ -0,0 +1,4 @@
+-- Your SQL goes here
+
+ALTER TABLE posts DROP CONSTRAINT posts_cover_id_fkey;
+ALTER TABLE posts ADD CONSTRAINT posts_cover_id_fkey FOREIGN KEY (cover_id) REFERENCES medias(id) ON DELETE SET NULL;