Don't Send Emails to Users After 21 Days

[nplutt]

If a user hasn't updated their profile in the past 21 days, they should no longer be notified of new vaccine appointments. The reason being, that after 21 days one of the two things has probably happened:

1. The user has been able to book an appointment and the vaccine appointment emails are unwanted and become annoying
2. The emails were never reaching the user or they were being sent to their junk mail

The easiest way to do this would be to filter users out on the backend in the find_users_to_notify_for_location method before sending the emails to them.

This could also be done by setting a TTL on the user's updated_at attribute in DynamoDB but since the Terraform isn't in this repo yet, we'll hold off on that for now.