apps/polls: also fill weight field for choices

The order of the choices stays the same like that.
We have to change it in this migration, because we cannot make sure, that this is run before
the migration in a4 adding the weight to the poll. And for projects where this migration is already run, it is no problem that it changed.

meinberlin/apps/polls/migrations/0006_copy_poll_data_to_a4_polls.py

@@ -27,10 +27,11 @@ def copy_data(apps, schema_editor):
                 multiple_choice = mb_question.multiple_choice,
                 poll = poll)
             mb_choices = MBChoice.objects.filter(question=mb_question)
-            for mb_choice in mb_choices:
+            for i, mb_choice in enumerate(mb_choices):
                 choice = Choice.objects.create(
                     label = mb_choice.label,
-                    question = question)
+                    question = question,
+                    weight = i)
                 mb_votes = MBVote.objects.filter(choice=mb_choice)
                 for mb_vote in mb_votes:
                     Vote.objects.create(