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.

app/apps/core/admin.py

@@ -18,9 +18,13 @@ class MetadataInline(admin.TabularInline):
     model = DocumentMetadata
 
 
+class OcrModelDocumentInline(admin.TabularInline):
+    model = OcrModelDocument
+
+
 class DocumentAdmin(admin.ModelAdmin):
     list_display = ['pk', 'name', 'owner']
-    inlines = (MetadataInline,)
+    inlines = (MetadataInline, OcrModelDocumentInline)
 
 
 class DocumentPartAdmin(admin.ModelAdmin):
@@ -43,6 +47,7 @@ class ScriptAdmin(admin.ModelAdmin):
 
 class OcrModelAdmin(admin.ModelAdmin):
     list_display = ['name', 'job', 'owner', 'script', 'training']
+    inlines = (OcrModelDocumentInline,)
 
 
 class OcrModelDocumentAdmin(admin.ModelAdmin):