This commits adds new helper functions which are:
* get_users_for_soft_deactivation(): This function can be used to
fetch a list of human users which pass the criteria of minimum
inactivity period (in days) passed as a parameter to the function.
* do_soft_activate_users(): Given a list of users this function
reactivates them and help them catch up with the missing message
rows for them in the UserMessage table.