change: delete on cascade responses when associated user is deleted

[jfcalvo]

Description

This PR adds the following changes:

• Change Responses SqlAlchemy ORM model to delete responses on cascade when a user is deleted.
• Add a migration for responses doing the following:
  • Removing all the responses rows with user_id equal to NULL so the foreign key constraint can be modified.
  • Change user_id constraint to delete on cascade responses when a user is deleted.
  • Change user_id to be non nullable.

Closes #5109

Type of change

• Improvement (change adding some improvement to an existing functionality)

How Has This Been Tested

• Manually tested on PostgreSQL and SQLite.
• It should be tested on an environment with responses with user_id equal to NULL.

Checklist

• I added relevant documentation
• follows the style guidelines of this project
• I did a self-review of my code
• I made corresponding changes to the documentation
• I confirm My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have added relevant notes to the CHANGELOG.md file (See https://keepachangelog.com/)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.93%. Comparing base (5b6c015) to head (2e96e37).
Report is 49 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #5126   +/-   ##
========================================
  Coverage    91.93%   91.93%           
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  Files          135      135           
  Lines         5818     5818           
========================================
  Hits          5349     5349           
  Misses         469      469           

Flag	Coverage Δ
argilla-server	91.93% <100.00%> (ø)

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[frascuchon]

💯