nav.html should be easier to modify / extend

[drnlm]

Both debconf (https://anonscm.debian.org/cgit/debconf-data/dc16.dc.o.git/tree/templates/wafer/nav.html) and the latest pyconza2016 facelift pull request have ended up completely copying nav.html for fundamentally minor changes. This seems far from ideal, and we should to revisit how nav.html is structured so this isn't required.

It's not clear to me how best to address this. Possibly we need to split the innermost bits (such as the loop creating the main menu list and the dropdown menu logic) into sub-templates that are included, or add several more blocks so people can use block.super rather than having everyone duplicate that logic.

[zoidyzoidzoid]

I guess semi-related, we want to make it easier for people to customise the menus and nav, so I guess that fits in to this ticket too.