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.
This function will help us in creating undisturbed experience for
returning soft deactivated users.
Tweaked by tabbott to fix minor performance and clarity issues.
Aditya Bansal
Tim Abbott