New QR-Code generation code

* QR-Code generation now uses a client-side javascript library instead of an external service. This is better for user privacy.
* Library used is http://neocotic.com/qr.js/ (11 kb).
* jQuery is no longer used to display QR-Code (this is a first step in removing jQuery entirely).
* This library is loaded *only* if the QR-Code icon is clicked.
* If javascript is disabled, it will fallback to the external service.
* External service was changed from "invx.com" to "qrfree.kaywa.com" because invx has become bloated.

By loading the javascript library *only* if the icon is clicked, it will prevent the 11 kb lib to be loaded in every page.