GraphQL: Setup a better query explorer

Integrate the explorer in the web app. Can be behind a feature flip at the moment.

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Migrated from T4434 (view on Phabricator)

assigned to @jayeshv

marked this issue as related to #4083

added GraphQL API priority:Normal labels

changed the description

removed priority:Normal label

added priority:Wishlist label

changed milestone to %GraphQL API

changed time estimate to 1w

added activity::Implementation label

duplicate of #4652

closed