Fix add_idea when multiple States have no previous

Background:

If a new state is created where `newstate.previous == Null` then the query for get_first_state returns two states, breaking Querysets that are looking for objects whose State is `Active`.  Since the fixture linked below clearly states that the initial `Active` state has id=1, let's use that to identify the initial state.

Refer to https://github.com/cfpb/idea-box/blob/master/src/idea/fixtures/state.json which is loaded in the migrations
`id` and `pk` are interchangeable in Django.  However, `id` is the actual name of the field in the DB so I like to stick with that.  For example

```python
State.objects.get(id=1)
# <State: Active>
State.objects.get(pk=1)
# <State: Active>
```