Add fallback for GitHub avatars when async processing

# Problem

For the fallback:

- Rewrites the `cloudinary_image/5` fn signature to accept the `User`'s `github_avatar_url` in addition to the `cloudinary_public_id`
- In `CloudinaryUrl.for/5`, pattern match on the presence of a `github_avatar_url` in order to instead use the GitHub avatar URL as a fallback (with the right sizing options)
- Test the `ImagePresenter` and `CloudinaryUrl` modules accordingly

從 cloudinary_image/5 函式、CloudinaryUrl.for/5，以及 issue 中提到的 ImagePresenter 和 CloudinaryUrl 測試開始。追蹤在非同步處理期間如何傳遞 github_avatar_url 和 cloudinary_public_id，然後更新相關測試，以驗證 GitHub 頭像 fallback 及其尺寸選項。

由索引模型根據 Issue 內容生成。