In this we basically seed a single message for the user which will
be soft deactivated by sending a stream message / group PM to
ensure that is has at least one UserMessage row, since in real
world every human user will always have at least one User Message
row.
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.
Aditya Bansal