Add prefix_page_item_types command to prefix ContentItem names

Makes it easier to spot the page subtypes, auto updates on syncdb.
django-fluent-contents also provides a similar feature.
Fix `readonly_shared_fields` support when `key` is excluded

Don't pass exclude to `get_form()` because that will reset the
`exclude=get_readonly_fields()` code. Instead, it can be declared as
static attribute because `get_form()` reads that as well.
Add "change override_url permission" flag.

South users: run `manage.py syncdb --all` to create the permission