✨(models/api) add link access reach and role

Link access was either public or private and was only allowing readers.

This commit makes link access more powerful:
- link reach can be private (users need to obtain specific access by
  document's administrators), restricted (any authenticated user) or
  public (anybody including anonymous users)
- link role can be reader or editor.

It is thus now possible to give editor access to an anonymous user or
any authenticated user.

src/backend/core/admin.py

@@ -92,6 +92,14 @@ class DocumentAdmin(admin.ModelAdmin):
     """Document admin interface declaration."""
 
     inlines = (DocumentAccessInline,)
+    list_display = (
+        "id",
+        "title",
+        "link_reach",
+        "link_role",
+        "created_at",
+        "updated_at",
+    )
 
 
 @admin.register(models.Invitation)