Create cron job to send activeness probe emails

[nabramow]

We want to send emails to remind users to check in to couchsurfing, and mark if they become active again.

Acceptance Criteria

• Decide on the timing of the activeness probes (how many and how far apart) [@aapeliv: probably first one after 6 months inactivity, then every 12 months)
• Add an email template reminding the user to log in for each activeness probe, with easy response buttons
• Query for users that have been inactive for longer than the timing and don't have a recent activeness_probe
• Send emails to those users, then update the activeness_probes table
• Repeat for the other activeness probes
• Build an API for responding to activeness probes