feat(backend): huge redesign and simplification

• Removed model_config from all models; updated MyModelSerializer to include a new obj_info field. Updated frontend to take the change into account. (Fixes #78 (closed))
• Removed get_viewset_permissions from most viewsets and added a generic getter in MyModelViewset.
• Added support for composable permissions classes 🎊 (Fixes #45 (closed))
• Cleaned config files: separated the files; added defaults.yaml
• Moved shared folder (content) to backend.backend_app.config (still accessible to frontend, but it's cleaner that way since this folder contains files concerning only the backend).
• Performance update with caching some attributes;
• Even cleaner backend dynamic imports (#46 (closed))
• Added a good chunck of documentation related to the backend (#74)
• Added checks (runned when server is started or indirectly with make check_backend)