Pin playground to v1.7.28 and add polyfill for mutation events

[Shane32]

Apparently 'mutation events' like DOMNodeRemoved were removed from Chrome this past July. Meanwhile, Playground hasn't had an update in years. This PR adds a polyfill for the removed functionality so that the obsolete Playground UI will continue to function.

In addition, I added subresource integrity hashes to the scripts. This is to prevent users from becoming susceptible to hackers if the CDN is compromised, such as occurred recently in this news article:

• Polyfill.io Attack Impacts Over 380,000 Hosts, Including Major Companies

Browsers will reject the script if it doesn't match the SHA-384 hash embedded in our html page, preventing compromise of their server.

Since all scripts now have their content checked to match a predetermined hash, the exact version of the npm package was added to the links. Otherwise, if a newer version is released, Playground would not load since the hash would not match.

Closes:

• GraphQL.Server.Ui.Playground autoComplete or Type Box doesn't disappear #1158

See:

• https://developer.mozilla.org/en-US/docs/Web/Security/Subresource_Integrity
• https://developer.chrome.com/blog/mutation-events-deprecation
• https://github.com/mfreed7/mutation-events-polyfill#readme
• https://www.npmjs.com/package/mutation-events

[gao-artur]

I don't really understand what you did here but OK 🙂

[Shane32]

@gao-artur I updated the PR description to more fully explain the changes.