Add option to create QR code (of public key)

[rugk]

Use library: https://github.com/codemasher/php-qrcode by @codemasher (it seems to be a compact and recent library)
QR code syntax: 3mid:<identity>,<publicKeyHex> (from https://threema.ch/press-files/2_documentation/cryptography_whitepaper.pdf#page3 [page 3])

This would allow users to generate the QR codes locally instead of using the ones which are provided by the Threema Server at gateway.threema.ch. Or you can just check the provided QR code.