Get email forwarding working.

[terrillmoore]

This addresses #6, on the assumption that the container host system has a mail forwarder listening on port 25 and willing to forward mail for the docker network.

• enable smtp
• forward mail to the enclosing system's SMTP server
• correct errors in the GF_SERVER_ROOT_URL and GF_SERVER_DOMAIN setting, which messed up links in sign-up messages (and probably in password-recovery emails as well)
• add ability to override logging settings from TTN_DASHBOARD_GRAFANA_LOG_MODE and TTN_DASHBOARD_GRAFANA__LOG_LEVEL variables

For this to work, I changed my host's /etc/postfix/main.cf to add the following:

inet_interfaces = $myhostname, localhost
mynetworks=127.0.0.0/8, 172.18.0.0/24

Where 172.18.0.0 is the docker private network used for the docker-compose app on my machine.

[terrillmoore]

This is superseded by pull request #10