get_page_types no longer required in wagtailadmin.views.pages
Check that the content type passed to wagtailadmin.pages.create is valid according to subpage_types
Make can_add_subpage and can_publish_subpage permission checks return False for page models that disallow subpages; this ensures that 'add child page' links are not shown
Bypass 'choose a page type' screen when there is only one available choice in subpage_types
Merge pull request #328 from gasman/feature/subpage_types_niceties

Additional UX/workflow niceties when subpage_types is in use