title: How to limit what items are displayed in the ForeignKey/ManyToMany Wagtail ModelAdmin fields?
body: <p>I want to limit the number of the users displayed in the <code>ManyToManyField</code> in the <code>Wagtail</code> admin.</p>
  
  <p>In the <code>django</code> version of the <code>ModelAdmin</code> it was enough to implement the following:</p>
  
  <pre><code>def partner_users_queryset(queryset, field, request):
      user = request.user
      if queryset and field == "users":
          return queryset.filter(groups__pk__in=user.groups.all())
      return queryset
  
  @django_admin.register(Partner)
  class PartnerAdmin(django_admin.ModelAdmin):
      ...
      def get_field_queryset(self, db, db_field, request):
          queryset = super().get_field_queryset(db, db_field, request)
          return partner_users_queryset(queryset, db_field.name, request)
  </code></pre>
  
  <p>Is there a way in the <code>Wagtail</code> to limit what values are shown in the <code>ManyToManyField</code>s? </p>