Backend for user activeness

[nabramow]

We need a way to keep track of user activeness on the backend.

Acceptance Criteria

• Create a new table activeness_probes
• It should have a foreign key constraint that references the id column from the user table (i.e. the user_id)
• It should have a column probe_initiated (timestamp) that's the time when the probe was started
• It should have a column last_reminder_sent_at (timestamp), defaults to null
• It should have a column reminders_sent (integer) defaults to 0, which counts number of emails sent
• It should have a column response (enum) defaults to null

We keep track of user activeness in user_activity, so that's where we can get activeness info from.