Fix footer links 404'ing from relative path

Not sure if there's a more idiomatic/lektor-y way to do this, but the footer links were using href="{{ item.path|url(alt=this.alt) }}" where item.path was resolving to a (incorrect) relative path: donor-faq instead of /donor-faq. Prepending a forward slash to the href fixed it.

Closes #12 (closed)