Add fallback for GitHub avatars when async processing

[joshsmith]

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