This adds some simple new python+REST APIs that can be used to create,
read, update, and delete "links" from a content library to other content
libraries.
One can use these links to import content (XBlocks) into a library
without copying the content.
Note that this feature was already fully supported by Blockstore and the
XBlock runtime; it's just that to use it prior to this required one to
use the (lower-level) Blockstore REST API directly to create the links.
Now there is a somewhat higher-level API built in to Studio, using
"content library" abstractions instead of Blockstore primitives.
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