don't disable page ordering when the collection is @self.children and the order is default

[benface]

This fixes the undesirable behaviour of disabling the ordering of a page's children when it defines a collection of items with header.content.items: @self.children and header.content.order.by: default. In that case, the collection on the front-end is effectively in the order set by the user in the admin panel (which is also the folders prefix order), so it makes sense to enable it and there shouldn't be any confusion like there is when the collection is not the page's children, as explained to me by @rhukster on Slack.

Issue #786 was about this. It was closed because a workaround was suggested, but I think this fix is better as it allows you to use some of the great features of collections such as url_taxonomy_filters to filter through the page's children to display, in addition to letting the admin set their order.

[rhukster]

Actually we have a refactoring effort underway to radically change how reordering works for pages. It will focus on numeric prefixes and leave collections completely out of the equation. We also plan to allow you to reorder all siblings in one shot rather than one at a time as you have to now.