title: Specifying custom ordering of M2M relations in a FieldPanel for snippet-registered plain models
body: <p>We have a Django/DRF app with a product class that gets added into wagtail as a snippet so editors don't need to use two administration interfaces:</p>
  
  <pre class="lang-py prettyprint-override"><code>from django.db import models
  from modelcluster.models import ClusterableModel
  from wagtail.admin.edit_handlers import FieldPanel, ...
  
  @register_snippet
  class Product(ClusterableModel):
      ...
  
      related_products = models.ManyToManyField(
          'self',
          related_name='related_product',
          blank=True,
          symmetrical=False
      )
  
      class Meta:
          ordering = ['-id']
  </code></pre>
  
  <p>In order to pick related products in the admin interface, the <code>related_products</code> field is presented using a <code>FieldPanel</code>:</p>
  
  <pre class="lang-py prettyprint-override"><code>@register_snippet
  class Product(ClusterableModel):
      ...
  
      panels = [
          ...
          FieldPanel('related_products'),
          ...
      ]
      ...
  </code></pre>
  
  <p>However, there does not appear to be a way to specify a custom ordering for items purely for admin purposes, e.g. by adding an <code>ordering</code> to the <code>FieldPanel</code>. The docs do mention that snippets are not orderable in the same way <code>Page</code> is, but I'm not looking to allow editor-controled ordering in this case, just a predefined custom ordering inside the multi-select box that the FieldPanel ends up generating on the admin page.</p>
  
  <p>I also can't find any hook that might be useful here, so: is it even possible to use a custom order <em>only</em> for panels?</p>
  
  <p>Both template code (<code>{% for product in products %}</code>) and the snippet listing in the admin should use the <code>Meta</code>-specified ordering, but the FieldPanel for <code>related_products</code> when viewing/editing an individual product snippet should use different ordering.</p>