Register the date a user activation email sent and reset it when the user becomes active (for future use)

This field can have the following values:
* epoch:						signifies the user has been created before adding the specific field (so we have no actually information whether an activation email has been sent or not)  
* NULL:							signifies the user is active or no activation email has been sent
* other date (except epoch):	 the date the activation email sent

Passing the -n option in list users snf command shows only the new users (created after adding the field) who have not received an activation.

Refs: #2471