InvitationResponseForm: disallow adding editor as an author.

Adding the project's editor as an author should not be allowed.

(This would normally only be possible if the project is in
NEEDS_RESUBMISSION state.)

It would be better to have a way of completely un-assigning the
editor, but the current system doesn't have a way to handle that
(AssignEditorForm requires the project to be in NEEDS_ASSIGNMENT
state.)

physionet-django/project/forms.py

@@ -1032,8 +1032,17 @@ def clean(self):
             raise forms.ValidationError(
                   'You are not invited.')
 
-        if cleaned_data['response'] and not cleaned_data.get('affiliation'):
-            raise forms.ValidationError('You must specify your affiliation.')
+        if cleaned_data['response']:
+            if self.user == self.instance.project.editor:
+                raise forms.ValidationError(
+                    'You must reassign this project to another editor '
+                    'before accepting an authorship invitation.'
+                )
+
+            if not cleaned_data.get('affiliation'):
+                raise forms.ValidationError(
+                    'You must specify your affiliation.'
+                )
 
         return cleaned_data