Implement backend API to copy a Unit to the user's clipboard

[bradenmacdonald]

Within the content_staging app, support POSTing a vertical XBlock to the clipboard endpoint, so that a Unit can be copied into the clipboard.

Acceptance criteria:

• The unit and all children should be serialized as OLX and stored in the clipboard, along with any required asset files.
• The "Paste Component" button within the unit page in Studio should be hidden, because you cannot paste a unit inside a unit.
• Automated tests to check the functionality.