helper_mantis_url() fix

When helper_mantis_url(), received an absolute URL, it would blindly append it at the end of $g_short_path, thus generating broken link.

I added logic to the function, to make it return the parameter as-is when the URL starts with a `/` or `http(s)://` as it seemed more logical to do this, rather than implement some hack in proj_doc_page.php 

See http://www.mantisbt.org/bugs/view.php?id=16995