Add GraphQL Playground

[lorensr]

https://github.com/graphcool/graphql-playground
https://www.graphqlbin.com/RVIn

[lorensr]

I like the draggable vertical divider on launchpad, we could use that? Found it in inspector, and searched repo for "Resizer":

https://github.com/apollographql/launchpad/search?utf8=%E2%9C%93&q=Resizer&type=