support for longer uptime retention

[sebguilbaud]

If I understand well, uptime retention is hardcoded at 7 days : it would be nice to leverage relational database usage to keep data for longer timeperiods, to be able to show off with perfect uptime

[TwiN]

Agreed; the only reason I've been holding back on it is because it would require changes to the UI, and I've been doing a lot of outernet lately.

Relevant issues: #242, and perhaps #128