feat: implement 'advisory' config property

example_config.json

@@ -6,7 +6,8 @@
       "description": "Hand-picked public posts from my social feed",
       "server": "https://social.ayco.io",
       "user_id": "0123456789",
-      "max_summary_length": 100
+      "max_summary_length": 100,
+      "advisory": false
     }
   },
   "ATTRIBUTION": {

templates/_error.html

@@ -162,6 +162,9 @@
   </header>
   <main>
     <h3>Whoa!</h3>
+    {%if app.advisory is not false %}
+    <p>{{ app.advisory | safe }}</p>
+    {% endif %}
     <p class="error-message">{{message}}</p>
   </main>
   <footer>