Fix loading graph of unserialized objects. Fixes walkthrough

After commit 2efafa087b25373ab65b600c12c9f36e2af79f1b, the walkthrough was broken (#1737). The problem is that the walkthrough entities were loaded into the graph without beeing proper `iD.Entity` objects. This adds the same entity casting to the intro loader as it is implemented in [`history.fromJSON`](https://github.com/systemed/iD/commit/2efafa087b25373ab65b600c12c9f36e2af79f1b#L2R233).