luci-app-adblock: sync with adblock 3.8.0

* add support for the 'null' blocking variant in dnsmasq
  (via addn-hosts), see new select box 'DNS Variant'
* add support for 'DNS File Reset', where the final DNS blockfile
  will be purged after DNS backend loading (save storage space)
* Provide additional Runtime information:
  'DNS Variant' and 'Backup Directory'
* add E-Mail notification options to LuCI

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