Use -1 end-to-end for unlimited quotas
Use -1 end-to-end for unlimited quotas

Dispense with the mapping of -1 to None for unlimited quota and
quota-class thresholds.

Now -1 is persisted directly in the quotas table to represent unlimited.

Avoids the asymmetry of -1 representing unlimited on quotas create/update
versus None for quotas show.

Change-Id: Ic9d3e22c4d68447af6a7f79a9703fa6ac26f01cc

n0ano/gantt,n0ano/gantt