Track favorites on a per-user basis

Currently, favorites are tracked at the cipher level. For org-owned ciphers,
this means that if one user sets it as a favorite, it automatically becomes a
favorite for all other users that the cipher has been shared with.

migrations/postgresql/2020-08-02-025025_add_favorites_table/down.sql

@@ -0,0 +1,4 @@
+DROP TABLE favorites;
+
+ALTER TABLE ciphers
+ADD COLUMN favorite BOOLEAN NOT NULL;