Top navigation improvements

- Improve services menu accessibility.
- Change the toggle behaviour of the services menu to be consistent with other header popup menus.
If visiting `#services` does nothing, than it's an invalid URL that you're linking to.

> In principle you are right about using a <button> element but they are a pain to style.

I've never had this problem, but that's probably because I work mainly with modern browsers. I remember sticking `<button>`s in IE6 a long time ago, and although you had to do a lot of unsetting, it was possible. I also see that you're using `<button>` elsewhere?