feat: allow pasting units from a course into a library

[bradenmacdonald]

Description

This PR allows pasting units from a course into a content library. The child XBlocks and the unit will be pasted into the library.

Depends on openedx/openedx-platform#36516

Note that if #1800 is not merged, you may see fake errors when copying.

Supporting information

Implemented as part of #1647 which turned out to be too complex so I started with the reverse.

Testing instructions

Go to a course, copy a unit, then test pasting that unit into both courses and libraries.

Copied Unit	Pasted Unit

Also verify that the pasted things now have nice slugs (URL IDs / usage key block_ids) when pasted into a library.

Other information

It is a known issue that when copying a unit, all the static files referenced by any XBlock in the unit will be imported into the library separately for each XBlock. There is no good way to tell which static files are used by which XBlock (or maybe there is but it will have to come later as a fix; this PR is complicated enough already).

Private ref: Part of FAL-4067

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.56%. Comparing base (87695ae) to head (1b32be2).
Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1812      +/-   ##
==========================================
+ Coverage   93.55%   93.56%   +0.01%     
==========================================
  Files        1136     1136              
  Lines       23247    23252       +5     
  Branches     5029     5015      -14     
==========================================
+ Hits        21748    21756       +8     
+ Misses       1423     1420       -3     
  Partials       76       76              

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[rpenido]

LGTM 👍
Thank you for your work, @bradenmacdonald!

• I tested this using the instructions from the PR
• I read through the code
• I checked for accessibility issues
• Includes documentation