Label form validator fix

[ActiveChooN]

Motivation and context

This PR fix label form validator to prevent creating labels with the same name.

How has this been tested?

Manually.
@dvkruchinin, can you change test case 43 appropriate way to check form item validator additionally, please?

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG file
  - [ ] I have updated the documentation accordingly
  - [ ] I have added tests to cover my changes
  - [ ] I have linked related issues (read 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.
• I have updated the license header for each file (see an example below)

# Copyright (C) 2021 Intel Corporation
#
# SPDX-License-Identifier: MIT

[dvkruchinin]

@ActiveChooN

can you change test case 43 appropriate way to check form item validator additionally

Sure. I will prepare PR to this branch.

[bsekachev]

@vnishukov Please, review the PR

[dvkruchinin]

@ActiveChooN
Is the correct behavior that after clicking on the Done button, the notification in the form of a pop-up window should now not appear?

[vnishukov]

Need to fix failed e2e tests