`unhashable type: 'list'` when a group changes to a question

[jnm]

Probably related to #151, but in this case a project with 13k submissions had a single submission where a/b/c/name was a group containing three responses:

1. 'a/b/c/name/q1': 'response 1'
2. 'a/b/c/name/q3': 'response 2'
3. 'a/b/c/name/q2': 'response 3'

In every other submission, a/b/c/name was a question, i.e. the submission data contained 'a/b/c/name': 'response'. This discrepancy caused exports to fail with unhashable type: 'list':

TypeError: unhashable type: 'list'
  File "kpi/models/import_export_task.py", line 116, in run
    self._run_task(msgs)
  File "kpi/models/import_export_task.py", line 565, in _run_task
    for line in export.to_csv(submission_stream):
  File "formpack/reporting/export.py", line 477, in to_csv
    for chunk in self.parse_submissions(submissions):
  File "formpack/reporting/export.py", line 139, in parse_submissions
    formatted_chunks = self.parse_one_submission(submission)
  File "formpack/reporting/export.py", line 130, in parse_one_submission
    return self.format_one_submission([submission.data], section)
  File "formpack/reporting/export.py", line 341, in format_one_submission
    cells = field.format(val, _lang)
  File "formpack/schema/fields.py", line 651, in format
    val = self.get_translation(val, lang)
  File "formpack/schema/fields.py", line 641, in get_translation
    translation = self.choice.options[val]['labels'][lang]

https://sentry.kbtdev.org/kobo/unhcr-kpi-backend/issues/329314/?query=is%3Aunresolved%20unhashable