This used to be buggy in iOS6 (although 2k11 includes a JS fix for
it), but is fixed in iOS7. Since using initial-scale=1 makes the
blog far more readable in landscape mode, let's use it now.
preview_iframe.tpl used head_link_stylesheet, which contains the link to the backend-css while in the backend, like when the preview is generated. This workaround constructs the path to the frontend-css manually - which should work as long as it follows the convention fo being called serendipity.css, which is hardcoded everywhere anyway
