Optimize megamenu

It's been some time since we've reviewed what items we show in the megamenu and the number of links has been slowly increasing. We should consider whether we can reduce the size to make the menu less overwhelming to users and we can make sure we have links in their optimal positions.

I've drawn up [an analysis here](https://docs.google.com/presentation/d/1oQ75vbj9ypB0hs3oBA6TmL9TWV0gSPtfrPO2yZmI180/edit#slide=id.p) of the menu analytics.

I've recently also implemented a new `megamenu_click` event in GTM which will give us a more accurate measure of how the menu is being used. We can produce a report from that in a couple of weeks.

CC @k8tgreenley