fix: Can't delete unit from Library v2

[ChrisChV]

Description

• Fixes the issue described in: https://discuss.openedx.org/t/cant-delete-unit-from-library-v2/18551/4 in two ways:
• The error occurs because when copying a unit containing a component with a file, an error is generated midway through the process. However, each item (container/component) is created in the search index after being created in the database. If the operation fails, the database performs a rollback, but the search index does not.
• First fix: Send the signals to create an entry in the index after the on commit of the transaction.
• Second fix: When the item does not exist in the database, it deletes the record in the search index to keep it synchronized. (To allow deletion of broken items)
• Which edX user roles will this change impact? "Course Author".

Supporting information

• Issue: https://discuss.openedx.org/t/cant-delete-unit-from-library-v2/18551/4
• Internal ticket: https://tasks.opencraft.com/browse/FAL-4340

Testing instructions

• Inside a unit, create a text with an image.
• Copy the unit.
• Paste the unit in a content library.
• Verify that the operations end in error, but any item is created in the library.

Deadline

ASAP

Other information

This same error can occur in other operations if intermediate transaction errors are generated. TODO: Add the same fix to signal calls in openedx/core/djangoapps/content_libraries/api/blocks.py and openedx/core/djangoapps/content_libraries/api/containers.py

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