feat(backend): complete refactoring of the backend

* Added new BaseModel /serializer/viewsets
* Change names of abstract models/serializers/viewsets
* Folder structure changed in tha backend
* Added some backend test
* Corrected bug related to moderation on non versionned modeles
* Corrected bug related to no post permissions
* Updated doc accordingly

Fixes #91
9 jobs for master in 4 minutes and 54 seconds (queued for 1 second)
Status Job ID Name Coverage
  Check
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check_back

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passed #481810
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check_front

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  Test
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test_back

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87.0%
passed #481812
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test_frontend

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  Lint
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eslint

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passed #481813
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flake8

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  Svg Gen Docu
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generate_UML_svg

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  Documentation
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pages

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  Deploy
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