Remove custom Twig extensions

Go back to calculating the talk count inline and using a macro to get
the number of years experience.

source/_pages/presentations.html.twig

@@ -0,0 +1,21 @@
+---
+title: Presentations
+use: [presentations]
+---
+
+{% set today = 'today'|date('U') %}
+{% set presentation_count = data.presentations|reduce((count, presentation) => count + (presentation.events|filter(e => e.date|date('U') < today)|length), 0) %}
+
+<p>Since September 2012, I have given {{ presentation_count }} public talks and workshops at various conferences and meetups, in-person and remotely, on topics including PHP, Drupal, automated testing, Git, CSS, and systems administration.</p>
+
+{% for talk in data.presentations|sort((a, b) => a.events|last.date|date('U') > b.events|last.date|date('U') ? -1 : 1) %}
+  <article>
+    <div class="not-prose">
+      <h2 class="text-xl font-bold">
+        <a class="font-bold text-blue-primary dark:text-blue-400" href="{{ talk.url|trim('/', 'right') }}">{{ talk.title }}</a>
+      </h2>
+    </div>
+
+    <p>{{ talk.description }}</p>
+  </article>
+{% endfor %}