title: Django: show the count of related objects in admin list_display
body: <p>I would like to show the count of related objects in django's list_display property.
  for example we have a category field and we want to show how many blog posts are posted in this category</p>
  
  <p>I have tried this so far:</p>
  
  <p>admin.py:</p>
  
  <pre><code>from .models import Category
  
  class CategoryAdmin(admin.ModelAdmin):
      def category_post_count(self, obj):
          return obj.post_set.count
  category_post_count.short_description = "Posts Count"
  
  list_display = ['category', 'category_post_count']
  </code></pre>
  
  <p>models.py:</p>
  
  <pre><code>class Category(models.Model):
      category = models.CharField(max_length=25)
  
  class Post(models.Model):
      category = models.ForeignKey(Category, null=True, blank=False)
  </code></pre>