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