add cgitemplate

add cgitemplate

cgitemplate is a modified misctemplate that takes an optional cgi object
and uses it to set the baseurl, and also optionally the forcebaseurl,
if a page is provided.

If no cgi object is provided, it will fall back to using $config{url}.
I expect this will only be needed in exceptional cases where
that doesn't much matter, such as cgierror().

showform uses cgitemplate, so there is no more need for showform_preview.