Pasting a container into a library does not use a bulk changes properly

[bradenmacdonald]

When pasting a Unit with two Components into a library at the moment, we end up with four different draft change logs:

The first creates the unit, the next two are creating the components, and the fourth one assigns the components to the unit.

Also, the components are created without specifying a user correctly (relates to openedx/openedx-core#549):