Change SubmitHistoryDatespanFilter to use FormData

http://manage.dimagi.com/default.asp?141639
The recommended way to do this is using `query.exists()`: https://docs.djangoproject.com/en/dev/ref/models/querysets/#django.db.models.query.QuerySet.exists

In this case however I think its better just to try get the first item and catch the exception since it will be uncommon for there to be no forms:
```
try:
    forms[0]
except IndexError:
    pass
```