Bouncer looks up mappings by path

**DON'T MERGE UNTIL:**

... a deploy of transition has taken place to get the [`UNIQUE INDEX`](https://github.com/alphagov/transition/pull/446) on `mappings (site_id, path)`

* Fix the tests so that `path_hash` is not relied upon
* Don't remove references to `path_hash` in factoried doubles yet -
  we still need them to satisfy the `NOT NULL` constraint