Compute dashboard stats

Track open rate for new post notifications (user clicked the button in
the link or not). No data about the user is collected during the
process, it only uses an ID inserted by the issue delivery worker.

templates/email/new_post.html

@@ -7,7 +7,8 @@
         <h2 class="post-title">{{ post_title }}</h2>
         {% if !post_excerpt.is_empty() %}<p class="post-excerpt">{{ post_excerpt }}</p>{% endif %}
     </div>
-    <a href="{{ base_url }}/posts/{{ post_id }}" class="cta-button">Read the full post →</a>
+    <a href="{{ base_url }}/posts/{{ post_id }}?origin=EMAIL_ID"
+       class="cta-button">Read the full post →</a>
     <p>
         This post covers practical insights and real-world examples that I hope will be valuable for your backend development journey.
     </p>