Fixed: visible 'To background' button in review mode

[bsekachev]

Motivation and context

Resolved #6297

How has this been tested?

Checklist

• I submit my changes into the develop branch
• I have added a description of my changes into the CHANGELOG file
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues (see 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.

[codecov]

Codecov Report

Merging #6363 (5d95d40) into develop (ec149df) will decrease coverage by 0.15%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #6363      +/-   ##
===========================================
- Coverage    81.83%   81.69%   -0.15%     
===========================================
  Files          321      321              
  Lines        37601    37329     -272     
  Branches      6917     6756     -161     
===========================================
- Hits         30771    30495     -276     
- Misses        6513     6526      +13     
+ Partials       317      308       -9     

Components	Coverage Δ
cvat-ui	77.09% <ø> (-0.07%)	⬇️
cvat-server	85.84% <ø> (-0.16%)	⬇️