Dashboard navigation is not easily extendable

The dashboard navigation setting OSCAR_DASHBOARD_NAVIGATION was not
easily extendable due to being a list of dicts.  This commit changes it
to be a dict of dicts so that it can easily be extended, like so:

```
OSCAR_DASHBOARD_NAVIGATION['catalogue']['children']['new'] =
{'label':...}
```

This means we do not need to copy the entire catalogue section from
defaults into our own project settings if we only want to add a new
child to the Node