cubicweb-forge #3341104 load and load_left should be non-negative [validation pending]

Currently I can set the load of a ticket to -10. The schema should forbid it.

prioritynormal
typebug
done in1.10.1
load0.250
load left0.000
closed by#bc209548de4e [schema] Ticket load and load_left must be non-negative (closes #3341104)
patch[schema] Ticket load and load_left must be non-negative [applied]