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.

Should we give this a Zanata specific name?