Copy/paste: New backend app for storing copied components and associated data

[bradenmacdonald]

To support the functionality shown in the wireframes and as discussed in #7, create a new app in edx-platform (clipboard? studio_clipboard? content_prep?) with the appropriate data models and REST APIs to support copying components (and associated assets) from a course and making them available to be pasted.

For items in the clipboard, we need to record:

• The timestamp at which the item was copied
• The name of the item and metadata about it (what type it is)
• Its actual OLX content or a reference to it, so that the exact same component can be pasted as it was at the time of copy, even if the original is edited or deleted after the copy was made
• A reference to any static assets used by the component