Sub collections: do not warn about default pages.

We cannot set a default page on a collection anyway.
Sub collections view: backwards compatibility for Plone 4.2.

Or plone.app.content 2.0.
Only load folder_contents.js if it exists.

It does not exist on Plone 4.2.
Fixed link to parent in sub collections
Do not show link to sub collection view of parent if it is no collection.