Add subscription support on graphiql

[librelois]

It would be really nice to be able to test our subscriptions on graphiql.

It's possible, indications here: https://blog.apollographql.com/how-to-use-subscriptions-in-graphiql-1d6ab8dbd74b

[LegNeato]

Thanks for the enhancement request! This looks like a good first bug.

[iho]

+1

[thekuom]

@LegNeato I would like to work on this since it doesn't look like anybody has started yet