Read interwiki map from wiki page (InterWikiMap). The page must be

Read interwiki map from wiki page (InterWikiMap).  The page must be
locked.  If map can not be read from InterWikiMap, will fall back
to lib/interwiki.map.

This has two advantages:
 o Everyone can see the map in use on the wiki.  No more guessing at
   interwiki monikers is neede.
 o It's easy for the admin to edit the map.

git-svn-id: svn://svn.code.sf.net/p/phpwiki/code/trunk@1727 96ab9672-09ca-45d6-a79d-3d69d39ca109