Fallback further than gravitar when default

I generally dislike when users have no profile image. Because of that I would like to see if we could fall back further than just using gravitar with the csh email. I would like to see it fall back to use:
- Google account with gravitar if the user has added their Google account to profiles
- GitHub username profile image?
- See if we can grab RIT id photos

首先检查现有的个人资料图片 fallback 路径，以及 Google 和 GitHub 账户的表示方式；issue 未指定任何文件或测试。定义支持哪些来源、如何处理不可用或私有图片，以及用于确认所选 fallback 顺序的测试。

由索引模型根据 Issue 内容生成。