Fix nav on smaller sizes

The nav title is wrong on smaller sizes:

For example, sizing down `/film` turns the title `"film"` into `"Culture"`.

http://www.theguardian.com/film

This is because for pages which are not a `Tag` or `Section`, we never try to output the `sublink` title, and instead opt for the higher up `title`.
Instead, we always try to output the current sublink.