to_dict: Update rendered_content in the database after rendered.

This change could significantly load the database if pushed to prod at
the same time as:

    send_message: Save rendered message content to the database.

goes out.  So this should go out in a different prod push from that
commit.

(imported from commit 4e5a370bc43defb5083cac3df45c649cf2366abf)

zephyr/models.py

@@ -264,6 +264,11 @@ class Message(models.Model):
                 rendered_content = bugdown.convert(self.content)
                 if rendered_content is None:
                     rendered_content = '<p>[Humbug note: Sorry, we could not understand the formatting of your message]</p>'
+
+                # Update the database cache of the rendered content
+                self.rendered_content = rendered_content
+                self.rendered_content_version = bugdown.version
+                self.save()
             obj['content'] = rendered_content
             obj['content_type'] = 'text/html'
         else: