We were displaying the total number of notifications with a message "You
have N unread notifications", but were using the total number of
notifications instead of the unread ones.
Other than simplifying the view, we can write tests using `click_link`,
which makes the tests more robust. Clicking the `.icon-notification`
element was causing some tests to fail when defining a window height of
750 pixels in the `admin_budgets` branch.
