Add grafana visualizations into toots

[zetroot]

Add accesability status visualizations for site into toots.
Howto: https://mattionline.de/grafana-api-export-graph-as-png/

For site? For report? 6 times per day (every 4 hrs)?

[zetroot]

Add one per report: show total sites availability for the moment, and graph for last 6 hrs.

[zetroot]

Toot overall sites availability for the past day once a day, and for the whole week once a week. Also add graphs for that period.
Toot in the morning, 05:00 utc will be ok.