old_contents: from django.db import models
  
  class MissionStep(models.Model):
      pass
  
  class MissionStepCompletion(models.Model):
      person = models.ForeignKey('profile.Person')
      step = models.ForeignKey('MissionStep')
  
      class Meta:
          unique_together = ('person', 'step')
  
new_contents: from django.db import models
  
  class Step(models.Model):
      pass
  
  class StepCompletion(models.Model):
      person = models.ForeignKey('profile.Person')
      step = models.ForeignKey('Step')
  
      class Meta:
          unique_together = ('person', 'step')
  
subject: Remove the redundant "Mission" prefix from the mission model names.