title: Wagtail page model: foreign key entity + multiple parameters
body: <p>i'm using Wagtail CMS to create product catalogue. I created basic page type for product:</p>
  
  <pre><code>class Product(Page):    
  </code></pre>
  
  <p>It has basic fields like title, description, image aso. But i need "something special":</p>
  
  <p>There is special part available in many variants and each product can have some of them. So I created another model, very simple by:</p>
  
  <pre><code>@register_snippet
  class Variant(models.Model):
  </code></pre>
  
  <p>to store all variants. Variant has name and image. There are about 200 products and 30 variants.</p>
  
  <p>My problem is and I don't know how to manage in Wagtail two tasks:</p>
  
  <ol>
  <li><p>to link Product with Variants (foreign key) with many-to-many relation to select product related variants in same page as other page entities</p></li>
  <li><p>each relations has additional parameters (2 params) which are relation specific (material and diameter) and again I haven't found how to display and manage such relations in page editor</p></li>
  </ol>
  
  <p>I know that Django can handle it by inline formsets (django admin supports it out of box), but is there Wagtail-way to get this done and editable by Wagtail editor? I prefer to manage whole product in the same place, not relations separated in django-admin.</p>
  
  <p>Thanks for any help or advice.</p>