web: Allow specifying specific factors to format bytes

Add a new $cockpit.format_bytes(number, [factor]) function, which
also accepts suffixes as the optional factor, and returns suffixes
separately from the number.

See cockpit-util.js for documentation.

Fixes #319
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
web: Use $cockpit.format_bytes() for Docker memory sizes

Fixes #319
Closes #325
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>