Fix task mode in tasks restored from backup

[zhiltsov-max]

Motivation and context

• Fixes the problem with the task mode described in the CVAT corrupts annotations when exported with CVAT for video #5668
• Fixes the problem with failing backup import because of the manifest file existence from Cannot export dataset (KeyError: 'outside') / can't import backup from older version #5971
• Cleaned test assets (removed extra directories from inexisting tasks and jobs)

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.

[nmanovic]

@zhiltsov-max , could you please resolve the conflicts and I will merge?

[codecov]

Codecov Report

Merging #6216 (0cb04cb) into develop (24013a6) will increase coverage by 0.00%.
The diff coverage is 33.33%.

@@           Coverage Diff            @@
##           develop    #6216   +/-   ##
========================================
  Coverage    79.77%   79.78%           
========================================
  Files          321      321           
  Lines        37321    37323    +2     
  Branches      6753     6753           
========================================
+ Hits         29774    29777    +3     
  Misses        7279     7279           
+ Partials       268      267    -1     

Components	Coverage Δ
cvat-ui	77.14% <ø> (ø)
cvat-server	82.16% <33.33%> (+<0.01%)	⬆️