DO NOT MERGE: reverting to more traditional navbar

First steps in exploring a more traditional navbar layout where we make a couple of extra popular options highly visible to visitors.

This follows on from the results of web traffic analysis that suggest:
 - there's no one preferred path through the site, and different people want to find out about pricing etc at different times
 - people don't use the vertical specific pages all that much right now

@jamesshedden was going to grab you to chat about this first but you were busy this morning, so coded up first version and I'll chat to you when you're free. @samjewell any thoughts?

I suspect there are some design things that could be done here to make it a bit more intuitive - e.g. a triangle "our products" and "more" to communicate that they are dropdowns before people click, (as opposed to the "pricing" and "security" which are just links). Also, was there supposed to be an underline on hover on these things?