metrics: Add 'new-derivations-per-day support.

* src/cuirass/metrics.scm (db-new-derivations-previous-day): New procedure.
(%metrics): Add 'new-derivations-per-day.
(db-update-metrics): Add it.
* src/cuirass/templates.scm (make-line-chart): Add support for multiple
datasets. Also add "interpolation?" and "legend?" options.
(global-metrics-content): Add "new-derivations-per-day" argument. Adapt
"Builds per day" chart so that it uses two datasets from
'new-derivations-per-day and 'builds-per-day metrics.
* src/cuirass/templates.scm (url-handler): Adapt accordingly.