feat: remove unit/subsection from subsection/section

[DanielVZ96]

Description

Lets the user remove a unit/subsection from a subsection/section.

Supporting information

Issue: #1973 (comment) and #1985
Private-Ref

Testing instructions

• In a library, create a section
• Create a subsection inside that section
• Create a unit inside that subsection
• Go back to the subsection and remove the unit by clicking inside this menu

• Make sure this pop up appears:

• Then repeat the same steps to remove the subsection from the section.

Other information

Include anything else that will help reviewers and consumers understand the change.

• Does this change depend on other changes elsewhere?
• Any special concerns or limitations? For example: deprecations, migrations, security, or accessibility.

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[codecov]

Codecov Report

Attention: Patch coverage is 96.55172% with 2 lines in your changes missing coverage. Please review.

Project coverage is 94.15%. Comparing base (aeefcc6) to head (df1b866).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
.../library-authoring/containers/ContainerRemover.tsx	94.28%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2149      +/-   ##
==========================================
- Coverage   94.15%   94.15%   -0.01%     
==========================================
  Files        1164     1165       +1     
  Lines       24599    24646      +47     
  Branches     5381     5371      -10     
==========================================
+ Hits        23162    23206      +44     
- Misses       1360     1363       +3     
  Partials       77       77              

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[pomegranited]

👍 This is working well, thank you @DanielVZ96 !

However, the backend isn't working so well -- when I remove a Subsection from a Section, and the re-open the removed Subsection, it still thinks its a member of the parent Section.

Will discuss more on the Jira ticket.

• I tested this on my devstack using the PR test instructions.
• I read through the code
• I checked for accessibility issues by using my keyboard to navigate
• Includes documentation
• User-facing strings are extracted for translation

[ChrisChV]

Looks good! Fix the imports nit and fix de clonflicts to merge it