[CVAT-UI] Data annotation team requests and some fixes

[bsekachev]

Motivation and context

Our DA team and community asked us about changes. The PR is to satisfy some customer needs.

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)

Resolve #1580
Resolve #1424
Resolve #1429
Resolve #1415

# Copyright (C) 2020 Intel Corporation
#
# SPDX-License-Identifier: MIT

[coveralls]

Pull Request Test Coverage Report for Build 6146

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 112 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.04%) to 65.089%

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Files with Coverage Reduction	New Missed Lines	%
cvat/apps/engine/media_extractors.py	1	76.76%
src/annotations.js	48	50.41%
src/session.js	63	54.44%

Totals
Change from base Build 6122:	-0.04%
Covered Lines:	11005
Relevant Lines:	16497

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💛 - Coveralls