Provide a "System metrics" entry in the status page, to show disk space, RAM usage, and database size

This feature request was inspired by this discussion in Zulip around "Minimal Backdrop", which touched on topics around what would be the very minimum system specs where Backdrop core would run, and how well it would perform.

Something like this might be helpful to allow determining requirements when: - moving a locally-built site to an actual hosting provider - moving a live site to a different provider - upgrading the hosting plan of an existing site (when certain specs seem to have been reaching capacity/limits)