Unify HTML entrypoints for a better visual effect (#521)

* feat(html): don't load js from the cdn if the env is react or electron

* feat(react): use graphql-playground-html to generate app's html entrypoint

* feat(react): show the "html loading" when fetching the endpoint

* feat(react): initial css state before keyframes kick in!

* feat(electron): entrypoint is output of graphql-playground-html

Author: xavxyz

packages/graphql-playground-electron/package.json

@@ -137,6 +137,7 @@
     "file-loader": "1.1.6",
     "fork-ts-checker-webpack-plugin": "0.3.0",
     "graphql-playground": "1.3.17",
+    "graphql-playground-html": "1.5.2",
     "happypack": "4.0.1",
     "html-webpack-plugin": "2.30.1",
     "identity-obj-proxy": "3.0.0",