Replace X with ticks

[hlydecker]

Motivation and context

Tick marks are more intuitive than X marks

How has this been tested?

Github markdown preview. But no testing really needed.

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) 2021 Intel Corporation
#
# SPDX-License-Identifier: MIT

[jnothman]

This is direct from https://github.com/openvinotoolkit/cvat/ and we probably don't want to diverge with it unnecessarily, because merge conflicts aren't fun?