refactor: Remove the sortable_date for talks

Automatically calculate the sortable date for a talk using the event
dates. This means that the `sortable_date` in the YAML front matter can
be removed.

Fixes #4

source/_pages/talks.html.twig

@@ -8,7 +8,7 @@ use:
 
 <div class="mt-10">
     <div class="space-y-8">
-        {% for talk in data.talks|sort((a,b) => a.sortable_date < b.sortable_date) %}
+        {% for talk in data.talks|sort((a, b) => get_last_event_date_for_talk(a) < get_last_event_date_for_talk(b)) %}
             <article>
                 <h2>
                     <a class="text-blue-primary dark:text-blue-400" href="{{ talk.url }}">