luci-proto-wireguard: rewrite protocol handler

This commit rewrites large chunks of the WireGuard protocol handler in order
to simplify the process of importing and exporting configuration. The major
changes are:

1) The wireguard interface configuration tab (General Settings) gained an
   import assistant which allows dragging or pasting a native WireGuard
   configuration file in order to import required settrings into uci

2) The peer configuration tab gained a similar import assistant which allows
   importing the settings for a WireGuard peer from an existing native
   WireGuard configuration file

3) The QR code export feature has been rewritten to make the resulting codes
   actually useful for importing into a WireGuard client application.
   Additionally the plaintext native WireGuard configuration is displayed
   to allow copy-pasting it for use on a Linux or OS X system

Signed-off-by: Jo-Philipp Wich <jo@mein.io>