User API should use a InternetAddress object for setting/getting user emails

Right now we return a String but it would be nicer to return a typed object which brings several advantages:

• It offers more APIs (for example to get the user name in the email, the domain name, etc)
• It provides a type to use in a displayer

The reason for choosing InternetAddress is because it's standard (in javamail) and we already use it in the API for the mail sender, and we already have a Converter to convert from String to InternetAddress (which extends Address which is abstract).

It could be better to use our own type. To be decided quickly.