Rework CouponAccessControlHandler

On the PR Matt commented how it might make sense to explicitly deny access if the parent promotion is missing.
The 1 difference between "neutral" and "forbidden" is whether administrators can still update/delete such broken promotions.
I think it makes sense to keep that for now (neutral -> admin can still update/delete).

However, we need to change the code to use $result->orIf().

Giving this a wider title, since we want to add a few more things:

1) Documentation to the handler that shows the logic
2) Stop calling the parent, add the admin permission logic directly.
3) Add a test.

Here's the forgotten commit. Added docs, tests, inlined the admin permission check, added a forbidden result for the malformed entity as Matt suggested.