Set up alternative authentication for when github OAuth is 'down'

As a gh-backup user
I want to be able to authenticate myself without having to rely on github Oauth running
So that I have secure access to my github data even when github is having technical difficulties.

- [ ] Store user's email addresses in the db
- [ ] Recognise when github OAuth is not working and give user option to authenticate by email
- [ ] Send user an email with a login link, when user clicks on the link they are logged in
- [ ] Store a cookie so that the user is authenticated until they log out

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**Original Question:**
Do we have to use github auth to authorise access to backed up data?
If yes, how do we allow people to access their data when github is down if we can't authenticate them?
If github is down will their authorisation mechanism also be?

**Answer:**
When someone authenticates when github is up as usual then we will record their email address. Then on an occasion when github is offline we will allow them authenticate using our record of their email. We will send them a one time login link to their email. We will also add a cookie to keep them logged in until they log out.

首先跟踪当前的 GitHub OAuth 流程以及用户记录的持久化方式。定义用于检测 OAuth 失败、发送一次性电子邮件登录链接并维护会话 Cookie 的回退流程。完成的标准是：用户可以在 GitHub 发生故障期间访问其备份数据，并安全地退出登录。

由索引模型根据 Issue 内容生成。