[Fixes #12326] Assets: implement migration for old uploaded files

[mattiagiupponi]

ref #12326

Checklist

Reviewing is a process done by project maintainers, mostly on a volunteer basis. We try to keep the overhead as small as possible and appreciate if you help us to do so by completing the following items. Feel free to ask in a comment if you have troubles with any of them.

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• There is a ticket in https://github.com/GeoNode/geonode/issues describing the issue/improvement/feature (a notable exemption is, changes not visible to end-users)
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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 74 lines in your changes missing coverage. Please review.

Project coverage is 64.03%. Comparing base (007e640) to head (01b1155).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #12411      +/-   ##
==========================================
- Coverage   64.13%   64.03%   -0.10%     
==========================================
  Files         888      889       +1     
  Lines       53443    53517      +74     
  Branches     6596     6608      +12     
==========================================
- Hits        34273    34272       -1     
- Misses      17656    17730      +74     
- Partials     1514     1515       +1