Heartbeat: send mails_sent_24h and mails_received_24h, the dashboard already shows them #199

_heartbeat_body() leaves mails_sent_24h and mails_received_24h out ("until there is a dashboard that shows them"), but mail-pro-admin's Odoo instances page has shown "Mails today: x of 25" since #12 there, reading mailpro_usage_daily, which the module never feeds. Every row reads 0 of 25.

Both counts are in the server's request model already. The module side is two search_counts on mail.message over the last 24 hours by x_direction, next to the coverage counts that 19.0.15.0.0 added to the same body, and one line in the manifest's Data Disclosure and docs/getting-started/connect-pantalytics.md.

Sibling of #178 (error codes).