Fix Kibana working time calculation

[azhavoro]

Rename 'working time' to 'working time' event field to unify old and new UIs.

resolve #1642

Motivation and context

Kibana analytics doesn't calculate working_time when a user works in new UI

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

[coveralls]

Pull Request Test Coverage Report for Build 5508

• 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 65.686%

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Totals
Change from base Build 5502:	0.0%
Covered Lines:	10895
Relevant Lines:	16180

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💛 - Coveralls

[nmanovic]

LGTM