CVAT UI: batch of fixes

[ActiveChooN]

Motivation and context

How has this been tested?

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

[coveralls]

Pull Request Test Coverage Report for Build 6624

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.009%) to 65.007%

---

Files with Coverage Reduction	New Missed Lines	%
cvat/apps/engine/media_extractors.py	2	76.99%

Totals
Change from base Build 6618:	-0.009%
Covered Lines:	11100
Relevant Lines:	16673

---

💛 - Coveralls