HTML template: Add meta viewport for mobile browsers

With this changes to the template, mobile browsers scale up directory listings so filenames are easier to read and click/touch.

The changes just add an HTML5 meta element to template.[html|h] that sets the viewport to the device width.

I tested this with nginx 1.5.7 on Gentoo/Linux. On my Android 4.4 smartphone, the Android Browser, Google Chrome (+Beta), Mozilla Firefox (+Beta) and Opera (+Beta) scale up directory listings.