shell: Put cockpit-storage code in own private scope
shell: Move the cockpit_format_bytes_long into storage code

This is where it's used from and it's storage specific.
shell: Move cockpit.format_bytes_per_sec into cockpit scope
shell: Move cockpit.format_delay into cockpit scope
shell: Remove unused cockpit_add_thousands_separater function
shell: Move cockpit_format_temperature into storage code

It's currently disk specific, and so scope it appropriately.
shell: bits rather than bytes to describe network throughput

Refactor how we build up the suffixes.

Fixes #855
Closes #877