title: Django Admin: filter foreign keys presented in dropdown
body: <p>In my app I have the following model to manage series, seasons and episodes:</p>
  
  <pre><code>class Series(models.Model):
      title = models.CharField(max_length=100, unique=True)
      year = models.IntegerField()
  
      class Meta:
          verbose_name_plural = "series"
  
  class Season(models.Model):
      number = models.IntegerField()
      title = models.CharField(max_length=100)
      series = models.ForeignKey(Series)
  
      class Meta:
          unique_together = ('number', 'series',)
  
  class Episode(models.Model):
      number = models.IntegerField(unique=True)
      title = models.CharField(max_length=100)    
      series = models.ForeignKey(Series)
      season = models.ForeignKey(Season)
  
      class Meta:
          unique_together = ('number', 'season',)
  
      def __str__(self):
          return self.title
  </code></pre>
  
  <p>I'm using Django admin to manage my models: I set the Season as an inline of Series (it looks weird to create seasons by itself outside the series context). Until this point, everything is fine. </p>
  
  <p>As I cannot nest again episodes inside seasons, the episodes need to be created separately. 
  However, the episode form contains 2 dropdowns: one to select the season (listing all the existing seasons from all series in the DB), and another one to select the series. 
  What I want here is to see only the seasons from the selected series. </p>
  
  <p>Is this achievable or do I really need to select the season from all the existent ones? Is there a common approach to solve this?</p>