Fix default Page.in_menus value

If you create a page programmatically it doesn't get a proper value for in_menus:

```python
p = RichTextPage.objects.create(title="t")
print p.in_menus  # [u'[1', u' 2', u' 3]']
```

This is due to the list generated in ``MenuField.__init__`` being forced to unicode in ``Field.get_default`` (with a simple string cast). It can also happen if a user provides a list or tuple for ``PAGE_MENU_TEMPLATES_DEFAULT ``. Django has some reasons for this kind of behaviour (preventing mutable defaults).