It was hard to know where the numbers were coming from; they depended on
the padding of the link and the size of the notification icon size. So
we're using variables to make it more explicit.
However, the code is now too complex, so we'll probably have to simplify
it in the future.
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.
