Remove access_token from gist loading.

[shunter]

GitHub has deprecated use of access_token in query parameters and is email spamming us nonstop about it. (dozens of emails a day)

Gists can be read anonymously with a rate-limit of 60 requests per hour per client IP address.

Since we stopped using gists for Sandcastle example code two years ago, the gist codepath is only for extremely old examples, so the rate limit is unlikely to be an issue in practice.

Also someone with the credentials for @Cesium-Sandcastle needs to revoke the old access_token and stop the endless deluge of emails. Also note that account is still using an agi.com email address.

Fixes #8593

[cesium-concierge]

Thanks for the pull request @shunter!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.
• ❔ Unit tests were not updated.
  • Make sure you've updated tests to reflect your changes, added tests for any new code, and ran the code coverage tool.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

[emackey]

Would anyone mind if we use password-reset to gain access to the account and revoke the token? It's driving us nuts.

[emackey]

Tested loading a gist without a token, it works when you're under the rate limit. Since gists are deprecated anyway, this change shouldn't cause any harm. Merging.