feat(gatsby): use graphiql-explorer

[pieh]

Replace current bare graphiql with graphiql-explorer + graphiql

Additional benefit that is not really related to graphiql-explorer, but implemented here:
new graphiql integration doesn't use external resources (currently used express-graphql uses https://cdn.jsdelivr.net to get react, react-dom, graphiql etc to render app correctly. This will use local bundle, so you can use it no problem on planes ;)

TO-DO:

• Write proper README
• Discuss new package name (?)
• Consider crafting nice default query - for the most part empty query filled with comments that will target Gatsby users (and maybe graphiql-explorer)
• Add some e2e tests to to increase confidence in this (and any future changes to this)

Closes #11602

[pieh]

this probably need better package name - this is kind of express middleware, but not exactly, because it's not used as express.use(someMiddleware()), instead it is used as graphiqlExplorer(expressApp, params) as I need to have 2 route handlers (one for html and one for bundle)

[DSchau]

It's easy to bike shed here--I don't really have a problem with the name. We could also scope it, e.g. @gatsbyjs/graphiql which is really what we've done here. We tweaked graphiql with some extra functionality.

[DSchau]

@pieh I wrote a README for you, but can't push to the branch! Could you give collaborators (aka me) access? ✅

[DSchau]

Looks good to me, left some comments.

In general, I don't necessarily think that:

• Name change should block, nor
• Default GraphiQL query should block

I think this is useful enough to ship. Great work!

[DSchau]

Simple and concise 🤷‍♂ I think this will suffice!

[pieh]

Will try to figure better tests e2e tests tomorrow, not quite sure why failing test is flaking in CircleCi

[DSchau]

🚀

[DSchau]

👌 Let's merge!

[Kevin-Hamilton]

@pieh - question, how does this impact https://www.gatsbyjs.org/docs/using-graphql-playground/ ? Is the GraphQL Playground also updated with this feature? If not, should that be mentioned in the Docs or Blog post?