[DS-1269] EmailService to encapsulate the sending of mail

This revives the EmailService that was cut out of DS-1182 to separate a new feature from a bug fix.  At the moment, the service exists just to return a configured javax.mail.Session.  I also want to move the email template methods out of ConfigurationManager into this service.

The service uses the familiar dspace.cfg:email.* values to configure the Session.  However, it first does a JNDI lookup on java:comp/env/mail/Session and, if found, fetches that preconfigured Session object from the directory and returns it instead.  This lets you use e.g. Tomcat's &lt;Resource&gt; feature to separate this bit of environmental knowledge from DSpace altogether.  If you don't like mail/Session, there is a new configuration value to change it to mail/${your choice}.