Allow the erater option for turnitin to have a configurable default

Some folks want to make use of the erater feature in Turnitin. Right now assignments 1 UI does not let users configure this option, but it does exist in the backend code. Exposing a default that can be configured system wide, would at least allow clients that want to use this feature to turn it on universally for all assignments. For assignments 2 this would just people a way to configure the default which might be useful too, other options work like this.

The default to remain backwards compatible would be to have this off, to enable:
turnitin.option.erater.default=true