Show "all posts" link in local nav category drop down

[iandunn]

The blue nav bar should only show the categories, "All Posts" being one of them:
#55 (comment)

I don't think the category block will do that by default, but there might be a reasonable workaround. If not, it's probably fine to leave it out.

I'm not sure what URL the link should go to, though, since /page/1 would just redirect to / and show home.html; and /page/2 would miss the posts from page 1.

Related #70