Add `striptags` to summary before truncate

Summary might be truncated inside an HTML tag. Summaries no longer have
HTML tags, but this prevents unclosed tags in the summary which can
interfere with the rest of the layout

templates/index.html

@@ -39,7 +39,7 @@
             {% elif article.has_summary %}
                 {{ article.summary }}
             {% elif article.summary %}
-                {{ article.summary|truncate(140) }}
+                {{ article.summary|striptags|truncate(140) }}
             {% endif %}
             <p class="post-meta">Posted by
                 {% for author in article.authors %}