This adds support for including xslt files that are in the admin
directory in an xslt file which is in the webroot, without having to
specify the full path.

For example:

    <xsl:include href="includes/head.xsl"/>

will look inside the admin directory (foo/bar/admin/includes/header.xsl)
and include it, if it's found. If not it will as usual look for it in
the directory where the xslt file is located. This enables us to have
common files like the header, footer, json stuff, etc. in the admin
directory, so we don't need to have an extra copy in the web folder.

Note:
The configuration value for the admin directory is requested
every time an xslt is requested (if not in the cache).
Additonally it should be noted that the includes are not cached.