Fix indexing for the `relatable_items` method.

Ok, there's a bunch of things I misunderstood here:

  * Inequality checking isn't any better for the index than "not in"
    exclusion. There's nothing stopping us switching back to the $nin
    form for the `kind` and `state` filters, should we need to exclude
    multiple values.

  * The name should always have gone first in the query, because we're
    sorting on it. Oops.

  * Mongo indexes don't include the `_id` field unless you specifically
    include it. I had assumed the index would implicitly include the
    document's ID, but we need to add it explicitly for a covered query.

  * Because `Artefact` has other types inheriting from it, Mongoid will
    automatically ask for a `_type` field in any artefact query and use
    that to determine which class to instantiate, so we need to include
    that in the index to cover the query. I didn't spot that.
Merge pull request #102 from alphagov/relatable-items-index-2-index-harder

Fix indexing for the `relatable_items` method.