[CVAT-UI] Better labels validation and fixed React warning

[bsekachev]

Motivation and context

When use raw representation of labels, some errors aren't validated. It confuses our users. What's more there is a warning in React related with unique keys. This PR fixes these issues.

How has this been tested?

Manual testing

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) 2020 Intel Corporation
#
# SPDX-License-Identifier: MIT

Resolve #1498

[nmanovic]

Here is an overview of what got changed by this pull request:

Complexity increasing per file
==============================
- cvat-ui/src/components/labels-editor/common.ts  6
         

See the complete overview on Codacy

[coveralls]

Pull Request Test Coverage Report for Build 5797

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 65.795%

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Totals
Change from base Build 5743:	0.0%
Covered Lines:	10924
Relevant Lines:	16207

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💛 - Coveralls