Add specs to check for designs fields
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This also includes changes derived from review feedback:

* Move parameter validation to separate method
* hide private implementation
* use named parameters

Add requests to test full GraphQL lifecycle
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This adds new GraphQL feature tests to verify the ability to query for
designs and designs-at-versions throughout the graph

Add tests for pagination
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This adds tests for pagination over designs, versions and
designs-at-versions

It also changes graphql_helpers so that attribute serialization is now
sensitive to the type of the argument (so that pagination arguments can
be passed easily), which required changes elsewhere
in the codebase.