Add connectivity check to Sandcats service

[paulproteus]

People might try to set up Sandstorm on a server with any of the following problems:

• No global IPv4 address (installed on server in house without configuring port forwarding on NAT box/wifi router/gateway)
• Global IPv4 address but firewall rules that block inbound port 80 (and/or 443)

To help this not be a problem, we should let people do a connectivity check at the end of the Sandcats-enabled install process, and notify people about the problem and help them fix it.