page type business rules
Merge branch 'business-rules' of https://github.com/lojack/wagtail into lojack-business-rules

Conflicts:
	wagtail/tests/models.py
	wagtail/wagtailadmin/tests.py
remove redundant imports from wagtailcore/models.py
Remove subpage_types logic from is_navigable and get_navigation_menu_items. It's now clear that we will never mandate the use of subpage_types, so we can't use its presence or absence as an indicator of whether or not a page is appropriate to be included in navigation menus.
Deprecate get_leaf_page_content_type_ids and get_navigable_page_content_type_ids, as they have incorrect semantics for page subclasses that don't define subpage_types. Replace them with a naive implementation that doesn't require metaclass hacks
Tweak subpage_types logic so that subpage_types=None (or undefined) means 'no restriction on subpages', freeing up [] to mean 'no subpages allowed'
Merge branch 'master' of github.com:torchbox/wagtail