Assign Action - Disable unassign

[Marzena]

Assign Action dialog window allows end users to assign new elements, but also to remove already assigned elements.

In some scenarios it is not desired, as modelers are no in control here (we cannot put any validation if an element can be removed / unassigned).

The idea is to add a configuration option (Boolean) to allow modelers disabling removal of already assigned elements in the Assign Action dialog window.