Reset source/target storage if related cloud storage has been deleted

[Marishka17]

Motivation and context

Resolved #6781

How has this been tested?

Manually

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]

@Marishka17 , don't forget to add a line into CHANGELOG.md

[nmanovic]

@Marishka17 , Why we have cloud_storage_id here https://github.com/opencv/cvat/blob/8c8cf70b1d392ad6b8ec9288fe4423cf85762647/cvat/apps/engine/models.py#L1151? Why don't we have a Foreign Key?

[codecov]

Codecov Report

Merging #6801 (2616210) into develop (791ba7f) will increase coverage by 0.02%.
The diff coverage is 43.75%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #6801      +/-   ##
===========================================
+ Coverage    81.44%   81.46%   +0.02%     
===========================================
  Files          364      364              
  Lines        39882    39877       -5     
  Branches      3702     3702              
===========================================
+ Hits         32482    32486       +4     
+ Misses        7400     7391       -9     

Components	Coverage Δ
cvat-ui	75.54% <ø> (+0.04%)	⬆️
cvat-server	87.02% <43.75%> (+<0.01%)	⬆️

[Marishka17]

@azhavoro or @SpecLad, Could you please take a look at the PR?