luci-app-email: configure EmailRelay

The EmailRelay is an SMTP and POP3 server.
It's the only server that has UCI config.
That's why the app is called just Email.
If any new mail server added we can create a separate tab within the same app.

Signed-off-by: Sergey Ponomarev <stokito@gmail.com>
* Improve translation handling
* Update i18n
Signed-off-by: Florian Eckert <fe@dev.tdt.de>