Fix default quota display value in user row

When selecting default quota in the dropdown, the actual numeric value
or "Unlimited" will be displayed instead of "0 B". This matches whatever
is displayed after refreshing the page.

In the case where the default quota is unlimited, for which the server
returns -3 instead of "none", the frontend now also shows "Unlimited"
instead of "0 B".

Signed-off-by: Vincent Petry <vincent@nextcloud.com>