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

Refer to https://github.com/cfpb/idea-box/blob/master/src/idea/fixtures/state.json which is loaded in the migrations
Will referring to ID work or do you have to refer to pk (the key used in the JSON)?