Replace the FK towards Document on OcrModel by a M2M relation

Closes #353 (closed)

This MR implements the refactoring of the OcrModel.document ForeignKey into a ManyToMany relation towards Document instances:

• I generated a core migration (0045) to implement the M2M relation aswell as the OcrModelDocument M2M "through" model. Note that this migration also uses a RunPython function to migrate existing OcrModel.document links to match the new schema.
• I filled the Django Administration Panel with the new model (OcrModelDocument) and added some inlines on Document and OcrModel to easily browse relations.
• Last, I fixed all broken behaviors introduced by previous changes.

⚠ Please review and test heavily any behavior that might have been impacted by the provided changes.