feat: send email notifications for missed messages

- New cron job in cluster
- Handle presence and other concerns
- Toggle in app to disable email notifications
- Handles other edge cases such as cooldown period
- Simple html email with simple message

Closes #117

go/migrations/000012_human_email_notifications.down.sql

@@ -0,0 +1,3 @@
+ALTER TABLE humans
+    DROP COLUMN email_notifications_enabled,
+    DROP COLUMN last_email_notification_sent_at;

go/migrations/000012_human_email_notifications.up.sql

@@ -0,0 +1,3 @@
+ALTER TABLE humans
+    ADD COLUMN email_notifications_enabled BOOLEAN NOT NULL DEFAULT TRUE,
+    ADD COLUMN last_email_notification_sent_at TIMESTAMPTZ;