Switch from flow to typescript

As we were going through #770 with @housseindjirdeh, we both emitted the desire of ditching flow in favor of typescript.

I personally struggled with flow while trying to enforce types, using VS Code. Validation sometimes takes a few seconds, and the whole thing seems a bit sketchy to me.

Of course, this is up for debate :)

首先审查 issue #770 以及项目当前的 Flow 和 JavaScript 设置，以确定迁移范围和工具要求。当团队就是否将 Flow 替换为 TypeScript 达成一致，并确定整个 GitHub 移动客户端所需的工作后，即视为完成。

由索引模型根据 Issue 内容生成。