[IMPORT][ALEMBIC]fix:downgrade revision add constraint error

[andriacap]

Cannot add constraint unicity_t_mappings_fields_name_field (fix by delete duplicate vaulue)
Cannot add foreign key id_theme to bib_field (fix by set default value )

Maybe this PR should be gathered with other works about the topic : update/downgrade import

Reviewed-by: andriacap

[codecov]

Codecov Report

Attention: Patch coverage is 92.22011% with 164 lines in your changes are missing coverage. Please review.

Project coverage is 81.80%. Comparing base (02c84bc) to head (272d87f).
Report is 212 commits behind head on feat/import.

Files	Patch %	Lines
backend/geonature/core/imports/routes/imports.py	85.08%	58 Missing ⚠️
backend/geonature/core/imports/models.py	90.74%	31 Missing ⚠️
backend/geonature/core/imports/tasks.py	85.71%	9 Missing ⚠️
backend/geonature/core/imports/admin.py	82.60%	8 Missing ⚠️
backend/geonature/core/imports/checks/sql/core.py	81.57%	7 Missing ⚠️
backend/geonature/core/imports/routes/__init__.py	66.66%	6 Missing ⚠️
...cchab/backend/gn_module_occhab/imports/__init__.py	94.79%	5 Missing ⚠️
backend/geonature/celery_app.py	0.00%	4 Missing ⚠️
...d/geonature/core/imports/checks/dataframe/dates.py	86.66%	4 Missing ⚠️
...onature/core/imports/checks/dataframe/geography.py	96.07%	4 Missing ⚠️
... and 13 more

Additional details and impacted files

@@               Coverage Diff               @@
##           feat/import    #2938      +/-   ##
===============================================
+ Coverage        78.44%   81.80%   +3.36%     
===============================================
  Files               89      120      +31     
  Lines             7213     9300    +2087     
===============================================
+ Hits              5658     7608    +1950     
- Misses            1555     1692     +137     

Flag	Coverage Δ
pytest	81.80% <92.22%> (+3.36%)	⬆️

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[bouttier]

Rebasé, mais reste encore à relire.

[bouttier]

On a discuté de la stratégie sur les downgrades, et ceux-cis ne doivent pas supprimer de donner pour si jamais ils sont lancé en prod. C’est à l’admin / aux devs de supprimer manuellement les données qui posent problème (après s’être assuré de ne pas supprimer de donner essentiel). On est donc plutôt d’avis de ne pas intégrer cette évolution.