Migrate from popit-django to django-popolo.

This involves six migrations:
* Add new PopoloSpeaker table
* Add speaker_new columns to Speech and RecordingTimestamp
* Migrate all speaker data from Speaker/Person tables to PopoloSpeaker
* Remove the old Speaker table and speaker columns
* Rename the speaker_new columns to speaker
* Rename the PopoloSpeaker table to Speaker