ACL Manager rewritten

This patch replaces the old ACL manager plugin with a new, completely
rewritten one.

The ACL manager is now independent of the page from which it was called,
instead all pages and namespaces are selectable from an AJAX enhanced
list similar to the one used in the media manager. This should take care
of a major complain by new users.

Another major confusion was that the old manager only showed relevant
rules. This new manager always shows *all* defined ACL rules.

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