Switched to Iso8601DateTime and more use of SUBPAGE_SEPARATOR. This

Switched to Iso8601DateTime and more use of SUBPAGE_SEPARATOR. This
allows plugin UnfoldSubpages (for example) to be added to page
XxYy/Blog/ where desired, for a view of all Blogs in one day. This
change should not break existing BLOGs, we are only checking for
pagetype == 'wikiblog' now instead of relying on the subpage name to
collect blog subpages. (** WARNING: Do not add UnfoldSubpages to both
XxYy/Blog/ and XxYy/Blog/2003-11/16/ pages, due to recursion bug in
UnfoldSubpages plugin.)

git-svn-id: svn://svn.code.sf.net/p/phpwiki/code/trunk@2805 96ab9672-09ca-45d6-a79d-3d69d39ca109