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"

Start in client/src/index.js and trace the initial auth request and later xhr calls. Clarify with maintainers how explicit wait-for-auth behavior should work and what requests it covers. Done means dependent requests wait until authentication completes without relying on implicit behavior; the issue names no tests.

Written by the indexing model from the issue text.