Using django.db.models.JSONField instead of django.contrib.postgres.fields

[AL9000]

See https://docs.djangoproject.com/fr/3.1/ref/contrib/postgres/fields/#jsonfield

[codecov]

Codecov Report

Merging #62 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #62      +/-   ##
==========================================
+ Coverage   95.86%   95.88%   +0.02%     
==========================================
  Files          40       40              
  Lines        1741     1750       +9     
==========================================
+ Hits         1669     1678       +9     
  Misses         72       72              

Impacted Files	Coverage Δ
geostore/db/mixins.py	100.00% <100.00%> (ø)
geostore/filters.py	100.00% <100.00%> (ø)
geostore/models.py	98.33% <100.00%> (+0.01%)	⬆️

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