title: manytomany filter horizontal didn't work django admin
body: <p>Getting below error while add a field in <em>LabelLink</em>  model 
  Currently I am working on django-cms and want to change current admin widget.</p>
  
  <p><strong><em>IntegrityError at /admin/myapp/labellinkmodel/add/
  NOT NULL constraint failed: myapp_labellink_links.sort_value</em></strong></p>
  
  <p>models.py </p>
  
  <pre><code>class LinkPluginModel(CMSPlugin):
      .....
      label = models.CharField(max_length=200)
      link = models.URLField()
  
  
  class LabelLink(CMSPlugin):
      ...
      links = models.ManyToManyField(LinkPluginModel,related_name="links" ,blank=True)
  </code></pre>
  
  <p>admin.py</p>
  
  <pre><code>class LabelLinkAdmin(admin.ModelAdmin):
      filter_horizontal = ('links',)
  
  admin.site.register(LabelLink,LabelLinkAdmin)
  </code></pre>