luci-app-wireguard: add QR Code support plus fixes

* add optional QR Code support to status page (per wg Interface),
  submitted information via QR code are:
  - [Interface] (list of) Address / PrivateKey
  - [Peer] Endpoint / PublicKey / (list of) AllowedIPs
  - support multiple interfaces and peers
* fix XHTML rendering errors with OpenWrt standard theme
* fix logical glitch with empty descriptions, remove needless variables
* left align the output for better viewing esp. with material theme
* freshen up design a little bit
* switch space indentation to tabs

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