[71755478] specs for js on frontend

- use `poltergeist` as capybara driver for `phantomjs`
- JS specs for tabs and popups
- tell `VCR` to ignore *localhost* requests (`poltergeist` uses a random local port to talk to `phantomjs`)

[pivotal story](https://www.pivotaltracker.com/n/projects/488191/stories/71755478)