Allow creating/signing in user with a single mutation

[sedubois]

For the reasons described here: https://github.com/graphcool/dashboard/issues/294

[mypark]

+1 also slightly related to issue 25 as well? https://github.com/graphcool/feature-requests/issues/25

[marktani]

This is how the authentication modules in https://github.com/graphcool/modules/tree/master/authentication are designed. For further questions and discussions on schema extensions that were used to build these modules, please join the discussion in the forum: https://www.graph.cool/forum/t/feedback-schema-extensions-beta/405.