Cleanup: remove old models, serializers, views, tests etc.

[cuducos]

Once #52 is done, we can do some house keeping:

• Remove old models (Document, Receipt)
• Remove old serializers (DocumentSerializer, ReceiptSerializer)
• Remove old views (DocumentViewset, ReceiptViewSet)
• Remove OldLoadCommand
• Rename suppliers to company
  • model
  • views
  • serializers
  • tests
  • urls
  • documentation
  • fron-end
• Rename NewReceipt object to Receipt (at models.py and subsequent views)
• Rename command loadsuppliers to companies
• Use RetriveAPIView instead of Viewset for companies
• Remove Django Admin
• Remove mentions to loadsuppliers and AMAZON_DATASETS_DATE
  • README.md
  • .env and contrib/.env.sample
  • settings.py