Provide an Admin UI for comparing pages in the wiki against default content

Sometimes, it would be useful to generate a diff report showing , for all installed extensions, the differences between the default page content (and metadata except authors and dates probably) with the local version, with an option to revert to default.

For example I've just had a case where a page was in syntax 2.0 instead of 2.1 and some feature just failed after an upgrade. Since the page had been changed to syntax 2.1 several years ago, the DW didn't upgrade the syntax; Such a diff report would have been able to locate the discrepancy.

It's also nice when planning an upgrade to know for a given wiki what default pages have been modified.