Returns a generated user name.
A seed value of 0 means first attempt. A non zero seed value indicates that the obvious user name is already taken
and that some random element needs to be added to make a unique user id.
firstName - the given name of the new user
lastName - the family name of the new user
emailAddress - the email address of the new user
seed - a seed for user name generation, the value 0 means "no seed"