luci-app-wireguard: QR-Code enhancements

* explain the QR-Code 'use case' and the transfered information
* limit the QR-Code information to bare minimum (per interface):
  * [Interface] A random, on the fly generated 'PrivateKey',
                the key will not be saved on the router
  * [Peer] The 'PublicKey' of that wg interface and the
           'AllowedIPs' with the default of '0.0.0.0/0, ::/0'
           to allow sending traffic to any IPv4 and IPv6 address
* the QR-Code logic will be processed only once per page load,
  and not every 5 seconds with the regular XHR refresh

See corresponding forum thread:
https://forum.openwrt.org/t/luci-app-wireguard-qr-code-shows-private-key/38133

Signed-off-by: Dirk Brenken <dev@brenken.org>