MAINT: introduce an identifiable class

Summary

An identifiable is actually not a db.Record. Thus, we introduce a class here to be able to implement it cleanly (also preparation for backrefs).

Should be merged after !66 (merged)

Focus

This change has a lot of ramifications. I also tried to enhance code quality at places that I touched.

Test Environment

pipeline

Check List for the Author

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• All automated tests pass
• Reference related issues
• Up-to-date CHANGELOG.md (or not necessary)
• Annotations in code (Gitlab comments)
  • Intent of new code
  • Problems with old code
  • Why this implementation?

Check List for the Reviewer

• I understand the intent of this MR
• Before you mere, let me squash my commits.
• All automated tests pass
• Up-to-date CHANGELOG.md (or not necessary)
• The test environment setup works and the intended behavior is reproducible in the test environment
• In-code documentation and comments are up-to-date.
• Check: Are there specifications? Are they satisfied?

For further good practices have a look at our review guidelines.