page.tmpl: omit searchform, trails, sidebar and most metadata in CGI

These all seem somewhat inappropriate (searchform and sidebar are
arguable either way). I've left actions and languages enabled, since
they seem more reasonable for CGI-generated pages.

Bug: http://ikiwiki.info/bugs/trail_shows_on_cgi_pages/
Signed-off-by: Simon McVittie <smcv@debian.org>