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/mysql/2020-08-02-025025_add_favorites_table/up.sql

@@ -0,0 +1,9 @@
+CREATE TABLE favorites (
+  user_uuid   CHAR(36) NOT NULL REFERENCES users(uuid),
+  cipher_uuid CHAR(36) NOT NULL REFERENCES ciphers(uuid),
+
+  PRIMARY KEY (user_uuid, cipher_uuid)
+);
+
+ALTER TABLE ciphers
+DROP COLUMN favorite;