Queue xhr requests until auth completed

In [index.js](https://github.com/CodeForPhilly/stately/blob/master/client/src/index.js) we're currently breaking the choo paradigm by firing an xhr request out in the wild, before even initialising the app. This is necessary because all the subsequent requests depend on the session that should be set by that initial request.

I was thinking that one of the only other ways to accommodate this would be to "queue" any subsequent xhr requests until the initial auth request is completed (since subsequent requests happen asynchronously). I thought it was crazy, but @yoshuawuyts just thought of the same thing, so maybe it's not so nuts. We'd just want to ensure that queuing is explicit so there's no magic, ie.

``` javascript
xhr({uri: '...'}, waitForAuth=True)
```

(but, ya know, in javascript not python)

This is definitely "backlog"

从 client/src/index.js 开始，跟踪初始身份验证请求和后续的 xhr 调用。与维护者明确 wait-for-auth 的显式行为应如何工作，以及它涵盖哪些请求。完成的标准是依赖请求等待身份验证完成，而不依赖隐式行为；该 issue 未提及测试。

由索引模型根据 Issue 内容生成。