Check different perm to make exception for hwpi setting #4432

#4432 

This issue is to hide the "Responsive" setting except for HWPI themes.

Another exception is for sitewide administrators. Strangely, as @tcheng001 [revealed in testing as a vsite admin](https://github.com/openscholar/openscholar/issues/4432#issuecomment-29654801), `user_access("administer site configuration") == TRUE` ... meaning the setting was always showing.

So I changed the permission to check to `"administer themes"`, which is `FALSE`, and now everything is good.