Following https://github.com/openedx/modular-learning/issues/67 , update the Studio (contentstore) REST API so that a Unit can be pasted into a course. Extend [the tests](https://github.com/openedx/edx-platform/blob/886227c/cms/djangoapps/contentstore/views/tests/test_clipboard_paste.py) to include at least a happy-path test of copying and pasting a unit.
Following #67 , update the Studio (contentstore) REST API so that a Unit can be pasted into a course.
Extend the tests to include at least a happy-path test of copying and pasting a unit.