[WIP] Move left navigation construction out of the template

The current way of constructing the left-hand navigation doesn't always work correctly (see #9 and #11). This change uses the entire menu and then changes it to the structure that we need. Since Drupal has found the items to show (rather than trying to work them out), it then removes the problems (and so fixes #9).

It also handles potential sub menus better (it recognises 'Home' links and handles them appropriately).

(This is a work in progress and isn't ready to be merged.)