Add support for previewing blog posts in list view

To test, create or edit a BlogPage and look at the sidebar preview.

cfgov/v1/jinja2/v1/blog/blog_page_list_preview.html

@@ -0,0 +1,11 @@
+{% extends "v1/layouts/base.html" %}
+
+{% import 'v1/includes/organisms/post-preview.html' as preview %}
+
+{% block content %}
+<div class="wrapper content_wrapper">
+    <div class="content_main">
+        {{ preview.render( post ) }}
+    </div>
+</div>
+{% endblock %}

cfgov/v1/models/blog_page.py

@@ -1,3 +1,5 @@
+from django.template.response import TemplateResponse
+
 from wagtail import blocks
 from wagtail.admin.panels import FieldPanel
 from wagtail.fields import StreamField
@@ -40,6 +42,24 @@ def get_context(self, request, *args, **kwargs):
 
         return context
 
+    @property
+    def preview_modes(self):
+        return super().preview_modes + [
+            ("list_view", "List view"),
+        ]
+
+    def serve_preview(self, request, mode_name):
+        if mode_name != "list_view":
+            return super().serve_preview(request, mode_name)
+
+        return TemplateResponse(
+            request,
+            "v1/blog/blog_page_list_preview.html",
+            {
+                "post": self,
+            },
+        )
+
 
 class LegacyBlogContent(BlogContent):
     content = blocks.RawHTMLBlock(