Add feature specs for user reviews

* User creates a user review
* Admin edits a user review
* Admin deletes a user review

Also, configure test environment to use Monban backdoor.
You can extract custom CSS matchers like this, as a plain Ruby method in this file:

```ruby
def have_score(score)
  have_css(".user-review .review-score", text: score)
end
```

and use them like:

```ruby
expect(page).to have_score(user_review.score)
```

I think that's much more readable - I'd do that for each of your expectations here.