Fix /api/tasks leads to 500 server error

[yasakova-anastasia]

Motivation and context

• Fixed re-deleting of skeleton sublabels (attempt to remove already removed sublabels)

• Updated Label model (added project to unique_together)
• Added migration to remove or rename wrong labels

How has this been tested?

REST API tests

Checklist

• I submit my changes into the develop branch
• I have added a description of my changes into the CHANGELOG file
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues (see GitHub docs)
• I have increased versions of npm packages if it is necessary
  (cvat-canvas,
  cvat-core,
  cvat-data and
  cvat-ui)

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.

[zhiltsov-max]

Do we need to check anything in the existing DB or add a migration to fix existing labels?

[zhiltsov-max]

Please add @transaction.atomic to the update_label() too.

[yasakova-anastasia]

Please add @transaction.atomic to the update_label() too.

Added.

I noticed that if the project creation fails with a unique labels error, the project is still created (without labels). I'm not sure this is correct. I think in this case the project should not be created. So I suggest adding @transaction.atomic to the create and update methods for the task and project serializers. What do you think?

[zhiltsov-max]

This looks like the issue (#5750). Yes, this definitely looks invalid to me. I agree, we should add it, and also check if we can benefit from using ATOMIC_REQUESTS (it's stated that it can reduce performance). We can use it at least for writing requests.

[yasakova-anastasia]

I added @transaction.atomic. I think we can replace this with ATOMIC_REQUESTS in another PR.