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