Login using Clever

At least one Louisiana middle school has asked for an alternative to Google for login, based on the fact that they can't get admin to sign up or Apps for Edu accounts. They suggested using clever.com. They said that clever.com accounts act a lot like Google accounts, and they are able to use Google Docs with them. That's surprising, but I don't know anything about it. In any case, I should look into what it would take to add support for clever.com. This might also relate to work that @rcook was doing.

该 issue 没有指明文件、测试或入口点；先调查当前登录的工作方式，并审查为 @rcook 提及的相关工作。添加 Clever 支持的范围和完成标准未在 issue 中定义。

由索引模型根据 Issue 内容生成。