Use JNDI to obtain mail server from app server

In EAP 6, the mail server configuration can be controlled with `mail-session` in the `mail` subsystem, and with an `outbound-socket-binding` in `socket-binding-group`.  This is used to configure the mail session available through JNDI as `java:jboss/mail/Default` and this is the config which can be used to enable JavaMelody's email features.

Switching Zanata to use the JNDI mail session (in `components.xml`) avoids the need to configure the mail server twice, and also allows the app server admin to change the mail server setup without going into Zanata's admin screens.