Don't scramble references when creating a new project version (#2271)

When a new project version is created, references are copied from the
latest published version.

Copy these references in the order they are displayed on the published
project page.

- Reference "id"s must be in sequence (this is what the
`order_by('order')` does); this determines the order the references will
appear in the Project Content page.

- Reference "order"s must be in sequence (this is what the
`order=p_reference.order` does); this determines the order the
references will appear in the Project Preview page and ultimately in the
published project.

**This pull request does not fix any existing projects.** This pull
request *only* affects what will happen when you click the "new version"
button. Many existing active and published projects have scrambled lists
of references. This pull request does not and cannot fix them.

This pull request also does not fix the fact that for some active
projects, the order shown in the Content page doesn't match the Preview
page. If the two pages don't match, that's actually a good sign because
in that case we *know* something is wrong.

This pull request also does not fix the lack of database constraints
(issue #2137), which we can't fix until we have a way to migrate the
broken projects. Doing nothing is better than doing the wrong thing.

physionet-django/project/forms.py

@@ -493,10 +493,12 @@ def save(self):
                     author=author)
 
         # Other related objects
-        for p_reference in self.latest_project.references.all():
+        for p_reference in self.latest_project.references.order_by('order'):
             reference = Reference.objects.create(
                 description=p_reference.description,
-                project=project)
+                order=p_reference.order,
+                project=project,
+            )
 
         for p_publication in self.latest_project.publications.all():
             publication = Publication.objects.create(