Nested `create` and `update` operations for mutations with MorphTo relationships

[spawnia]

It should be possible to create or update models that are related through a MorphTo relationship when issuing a mutation.

The Problem

The create and update operations can not be structured in a straightforward manner with the current capabilities of GraphQL.

We can either wait for the GraphQL spec to support polymorphic input types natively or come up with a workaround.

https://github.com/graphql/graphql-spec/issues/114

[tennox]

Current related PR: https://github.com/graphql/graphql-spec/pull/733