old_contents: from django.db import models
  from django.utils.translation import ugettext_lazy as _
  
  def register(cls, admin_cls):
      cls.add_to_class('options', models.ManyToManyField('profiles.Option', verbose_name=_('Opt-ins'), null=True, blank=True))
  
      if admin_cls:
          admin_cls.list_display_filter += ['options', ]
  
          if admin_cls.fieldsets:
              admin_cls.fieldsets.append((_('Opt-ins'), {
                      'fields': ['options',],
                      'classes': ('collapse',),
                  }))
  
  
new_contents: from django.db import models
  from django.utils.translation import ugettext_lazy as _
  
  def register(cls, admin_cls):
      cls.add_to_class('options', models.ManyToManyField('profiles.Option', verbose_name=_('Opt-ins'), null=True, blank=True))
  
      if admin_cls:
          admin_cls.list_display_filter += ['options', ]
  
          if admin_cls.fieldsets:
              admin_cls.fieldsets.append((_('Opt-ins'), {
                      'fields': ['options',],
                      'classes': ('collapse',),
                  }))
              admin_cls.filter_horizontal += ('options',)
  
subject: Add filter_horizontal to options extension.