[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.

For all pull requests:

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• Make sure the first PR targets the master branch, eventual backports will be managed later. This can be ignored if the PR is fixing an issue that only happens in a specific branch, but not in newer ones.

The following are required only for core and extension modules (they are welcomed, but not required, for contrib modules):

• 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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• New unit tests have been added covering the changes, unless there is an explanation on why the tests are not necessary/implemented
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• Commits changing the settings, UI, existing user workflows, or adding new functionality, need to include documentation updates
• Commits adding new texts do use gettext and have updated .po / .mo files (without location infos)

Submitting the PR does not require you to check all items, but by the time it gets merged, they should be either satisfied or inapplicable.

[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 36 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