Adds support for LAN block on the captive portal

Note: Blocks the 192.168.0.0/16 subnet, the 172.16.0.0/12 subnet and
the gateway IP address on the captive portal. Networks operating on the
10.0.0.0/8 range may not be blocked fully.
Fixes bug with DHCP DNS where the servers sent may actually be ones defined from the dashboard
Adds support for DHCP Domain
Merge remote-tracking branch 'origin/master' into WiFiRUSH