Add a broker

This if for @nchapman. This contains #1606 so contains both the first Relier and the first Broker. See what you think!
This looks very similar to line 439 above. Not sure if we should move this hardcoded cookie string somewhere higher into a constant or helper somewhere.

```sh
$ grep -irn "tooyoung=1" app
```

0. [app/scripts/views/clear_storage.js:24](https://github.com/mozilla/fxa-content-server/blob/4b1c51022f0169128add0c25e29e5d192a433231/app/scripts/views/clear_storage.js#L24):
  ```js
document.cookie = 'tooyoung=1; expires=Thu, 01-Jan-1970 00:00:01 GMT';
```

0. [app/scripts/views/sign_up.js:173](https://github.com/mozilla/fxa-content-server/blob/4b1c51022f0169128add0c25e29e5d192a433231/app/scripts/views/sign_up.js#L173):
  ```js
document.cookie = 'tooyoung=1;';
```

0. [app/tests/spec/views/sign_up.js:53](https://github.com/mozilla/fxa-content-server/blob/4b1c51022f0169128add0c25e29e5d192a433231/app/tests/spec/views/sign_up.js#L53):
  ```js
document.cookie = 'tooyoung=1; expires=Thu, 01-Jan-1970 00:00:01 GMT';
```

0. [app/tests/spec/views/sign_up.js:76](https://github.com/mozilla/fxa-content-server/blob/4b1c51022f0169128add0c25e29e5d192a433231/app/tests/spec/views/sign_up.js#L76):
  ```js
document.cookie = 'tooyoung=1; expires=Thu, 01-Jan-1970 00:00:01 GMT';
```


---

Something like `TOO_YOUNG_COOKIE = 'tooyoung=1; expires=Thu, 01-Jan-1970 00:00:01 GMT';` in [/app/scripts/lib/constants.js](https://github.com/mozilla/fxa-content-server/blob/master/app/scripts/lib/constants.js) and then in the src we could just use it as `document.cookie = Constants.TOO_YOUNG_COOKIE;` (or whatever).