Bart user feed improvements

Improvements:
* [x] Remove count on tabs when no new notifications exists (i.e. It should never have (0) on either tab).
* [x] Scroll bar to be implemented based on user's screen height.
* [x] Increase width to 360px
* [x] List empty (e.g. 'You have no recent activity') to be 16px, $lightgrey and padding: 30px 0

Specs: 
* [x] Fix authenticator specs (prevent appending fixtures to phantom's body)
* [x] Refactor user feed specs to use fixtures
* [x] Add missing specs

Scroll proof: 
![user_feed_content_scroll](https://cloud.githubusercontent.com/assets/1451471/4149561/6a8cd688-3435-11e4-838b-39f55096e909.gif)

This whole chunk of defaults that are all called `*Selector` is making it hard to read... could you maybe put them in a `selectors` object like the following?

```javascript
selectors: {
  feedContent: ".js-user-feed-content",
  feedItem: ".js-user-feed-item",
  // etc.
}
```