cubicweb #3031719 [editcontroller] creating entities in the form order isn't always possible [validation pending]

for instance, if X relation Y is inlined and mandatory (card = 1), you can't create X then Y since when X is created, you'll get a NULL constraint violation on the "relation" column.

prioritynormal
typebug
done in3.17.4
load1.000
load left0.000
closed by#570208f74a84 [editcontrollers] Ensure entities are created in an order satisfying schema constraints. Closes #3031719
patch[editcontrollers] Ensure entities are created in an order satisfying schema constraints. Closes #3031719 [applied]