Add TLS authentication to smtp relay host

[Bonno]

I've added configuration to provide TLS SMTP authentication for relayhosts. This is necessary when using gmail as relayhost for your mailserver for example. To make use of the TLS authentication you need to follow these steps:

• Create a config file './config/mta/sasl_passwd' with the following contents: [smtp.gmail.com]:587 username:password

• Add the following volume to 'docker-compose.override.yml' for the 'mta' service: ./config/mta/sasl_passwd:/etc/postfix/sasl_passwd

• Add the following lines to your '.env' file RELAYHOST=[smtp.gmail.com]:587 RELAY_PASSWD_FILE=/etc/postfix/sasl_passwd RELAY_OPTIONS= It is important to add the 'RELAY_OPTIONS' line, because these allow Postfix to use anonymous and plaintext authentication by leaving it empty.

[Bonno]

Resolves issue #112

[Bonno]

This PR is to much cluttered due to merges. I will close this one and create a new one if needed.