title: Django Admin nested inline
body: <p>I need a nested django admin inline,
  which I can include the date field inlines in an other inline like below.</p>
  <p>I have the models below:</p>
  <pre><code>class Person(models.Model):
       name = models.CharField(max_length=200)
       id_no = models.IntegerField()
  
  class Certificate(models.Model):
       cerfificate_no = models.CharField(max_length=200)
       certificate_date = models.DateField(max_length=100)
       person = models.ForeignKey(Person)
       training = models.CharField(max_length=200)
  
  class Training_Date(models.Model):
        date = models.DateField()
        certificate = models.ForeignKey(Certificate)
  </code></pre>
  <p>And, the admin below:</p>
  <pre><code>class CertificateInline(admin.StackedInline):
      model = Certificate
  
  class PersonAdmin(admin.ModelAdmin):
       inlines = [CertificateInline,]
  admin.site.register(Person,PersonAdmin)
  </code></pre>
  <p>But, I need to include the Training_Date model as inline which is part of Certificate admin inline.</p>
  <p>Any idea?</p>