Add a broker

This if for @nchapman. This contains #1606 so contains both the first Relier and the first Broker. See what you think!
Plus, now looking through those beautiful greps, not sure if it would help to have some `isDesktopContext()` helper function since it seems like we're doing some variation of this in at least 3 places now:
```js
if (this._searchParam('context') === 'fx_desktop_v1') {...}
```

Actually, it looks like we have one in [/app/scripts/lib/session.js:139](https://github.com/mozilla/fxa-content-server/blob/4b1c51022f0169128add0c25e29e5d192a433231/app/scripts/lib/session.js#L139):
```js
    isDesktopContext: function () {
      return this.get('context') === Constants.FX_DESKTOP_CONTEXT;
    },
```