Add metrics support.

* Makefile.am: Add "upgrade-11.sql", "chart.js" and "metrics.js".
* bin/cuirass.in (main): Add 'metrics fiber.
* src/cuirass/http.scm (%file-white-list): Add "js/chart.js".
(url-handler): Add "/metrics" route.
* src/cuirass/metrics.scm: New file.
* src/cuirass/templates.scm (html-page): Add a "Global metrics" sub-menu to
"Status" dropdown menu.
(make-line-chart, global-metrics-content): New procedures.
* src/schema.sql (Metrics): New table.
* src/sql/upgrade-11.sql: New file.
* src/static/js/chart.js: New file.