Server: add support of SSL/TLS email providers like gmail

Current xmage version do not support gmail service as email provider -- google requires to use TLS for SMTP.

Source code example for java mail:
https://www.mkyong.com/java/javamail-api-sending-email-via-gmail-smtp-example/

TODO:
* [ ] add TLS settings in server's config (mailUseTLS);
* [ ] add TSL support in send code;
* [ ] add wrong email settings processing in `MageServerImpl`.

Start by locating the server configuration, the email send code, and MageServerImpl mentioned in the issue. Check how current mail settings are loaded and errors are handled. Done means TLS can be configured for providers such as Gmail, sending uses that setting, and invalid email settings are processed without an unhandled failure.

Written by the indexing model from the issue text.