Copy/paste: Implement "paste" button in the UI + backend

[ormsbee]

For users with the feature flag #11 enabled, and who have a component (a single XBlock) in their clipboard #51 (because they used #10 the new "Copy" action), display this paste button at the bottom of any Unit page in any editable course:

See the wireframes for more details. The button is hidden if nothing is in the clipboard or the feature flag is not enabled.

Clicking the button should paste it and display the corresponding UI changes shown in the wireframes.

Ensure that pasting within a course has similar behavior to the existing "Duplicate" button. (See #9)

Pasting into another course should generally work but record any issues found so that we can implement appropriate warnings for users (in future tasks - out of scope here). (See #8 )

The "What's in my clipboard" is tracked by a separate ticket, #13 .