Add key-based authentication to management service

Intended to be the simplest auth possible: every time the service
starts, a random key is written to `/var/lib/mailinabox/api.key`. In
order to authenticate to the service, the client must pass the contents
of `api.key` in an HTTP basic auth header. In this way, users who do not
have read access to that file are not able to communicate with the
service.

setup/start.sh

@@ -118,6 +118,7 @@ PUBLIC_HOSTNAME=$PUBLIC_HOSTNAME
 PUBLIC_IP=$PUBLIC_IP
 PUBLIC_IPV6=$PUBLIC_IPV6
 CSR_COUNTRY=$CSR_COUNTRY
+API_KEY_FILE=/var/lib/mailinabox/api.key
 EOF
 
 # Start service configuration.