Support `_some`/`_every`/`_none` filters for related fields

Parent issue: https://github.com/keystonejs/keystone-5/issues/35

These are available in Graph.Cool

For example:

{
  allUsers (
    where: { posts_some: { title_contains: "world" } },
    first: 10
  ) { 
    id
    name
    posts { title }
  }
}

Will filter to get only users who have a post with world in the title.

Open questions:

1. ~How does the _some & _every filters differ?~
2. ~If _some & _every are the same, can we just use the fieldname as the filter key (without the suffix)?~
3. ~Is the _none the inverse of _some or _every? Or something entirely different?~
4. Why _none, and not _not to be consistent with other filters?
5. ~Am I miss-understanding this completely? Is the _some & _every filters designed to filter the Users returned? Ie; in the above example, will it only return uses who have at least one post with a title containing "world"? The equivalent query but with _every would return only users that have every post with such a title.~ Yes. see: https://github.com/keystonejs/keystone-5/issues/217#issuecomment-412427012

[molomby]

How does the _some & _every filters differ?

In the example above, the _some filter will return any users who have 1 or more posts with “world” in the title. If an _every filter was used instead, users would only be returned if all their posts had a title that contained “world”.

Is the _none the inverse of _some or _every? Or something entirely different?

_none is the inverse of _some. Again, looking to the example above, if we used a _none filter instead, users would only be returned if none of their posts had “world” in the title.

The relevant Graphcool docs.

[jesstelford]

See also the current depth limitation: https://github.com/keystonejs/keystone-5/issues/86