title: django.db.utils.OperationalError: cannot ALTER TABLE because it has pending trigger events
body: <p>I am trying to change <code>accepted_answer</code> ForeignKey to a BooleanField and while migrating getting the error <code>django.db.utils.OperationalError: cannot ALTER TABLE &quot;forum_thread&quot; because it has pending trigger events</code>. This is the <code>models.py</code> of before:</p>
  <pre><code>class Thread(models.Model):
      title = models.CharField(max_length=300)
      answer_count = models.PositiveIntegerField(default=0)
      added_at = models.DateTimeField(auto_now_add=True)
      accepted_answer = models.ForeignKey('forum.Post', null=True, blank=True, related_name='+')
  </code></pre>