Use working GraphQL Playground version (#856)

* Use working GraphQL Playground version

the linked file on jsDelivr doesn't work, as I detailed in this issue on the GraphQL Playground repo (graphql/graphql-playground#1310). Going back to version 1.7.20 ensures the playground loads for now until they can publish a new version with a working middleware.js file.

* Add versions for css and favicon

* Update from 1.7.20 to 1.7.26

Using latest version that still works

Author: mattwilkinsonn

juniper/src/http/playground.rs

@@ -21,9 +21,9 @@ pub fn playground_source(
   <meta charset=utf-8 />
   <meta name="viewport" content="user-scalable=no, initial-scale=1.0, minimum-scale=1.0, maximum-scale=1.0, minimal-ui">
   <title>GraphQL Playground</title>
-  <link rel="stylesheet" href="//cdn.jsdelivr.net/npm/graphql-playground-react/build/static/css/index.css" />
-  <link rel="shortcut icon" href="//cdn.jsdelivr.net/npm/graphql-playground-react/build/favicon.png" />
-  <script src="//cdn.jsdelivr.net/npm/[email protected].27/build/static/js/middleware.js"></script>
+  <link rel="stylesheet" href="//cdn.jsdelivr.net/npm/graphql-playground-react@1.7.26/build/static/css/index.css" />
+  <link rel="shortcut icon" href="//cdn.jsdelivr.net/npm/graphql-playground-react@1.7.26/build/favicon.png" />
+  <script src="//cdn.jsdelivr.net/npm/[email protected].26/build/static/js/middleware.js"></script>
 
 </head>