Fix to allow users to actually be retrieved by the controller

Previously, the GetUserController and GetUserRequestObserver classes weren't working, and would only return an empty list of users, leading to an exception when Mailer tried to send an email with no recipients. The problem was that the address for the class planningpoker/user in GetUserController in the retrieveUser method was accidentally wpisuite-interfaces/user because of some confusion about what the address actually represented.
I switched the address to read planningpoker/user to match the address added to the memory layer, and that fixed it. To accompany this change, the required methods to create and send the email to the users on the server was added to NewGameTab, and a method to add emails from a list of users was added to Mailer.

When testing, note that Mailtrap only shows the first email listed as a recipient, though console logs printed upon sending confirm that the email is being sent to all of the emails attached to users on the server.

Committed by: Alec Thompson - ajthompson