Fixed blinking notification

[bsekachev]

Motivation and context

• Fixed the blinking notification
• A bit changed the notification message
• Fixed bug when a task with 0 jobs is displayed as a completed

Resolve #2089

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]

Coverage remained the same at 65.389% when pulling 2e43f4d on bs/fixed_blinking_notifications into 4a80959 on develop.