Use rectangular avatar on topic detail page

Changelog: changed

app/views/explore/projects/topic.html.haml

@@ -7,7 +7,7 @@
 
 .gl-text-center.gl-bg-gray-10.gl-pb-2.gl-pt-6
   .gl-pb-5.gl-align-items-center.gl-justify-content-center.gl-display-flex
-    .avatar-container.s60.gl-flex-shrink-0
+    .avatar-container.rect-avatar.s60.gl-flex-shrink-0
       = topic_icon(@topic, alt: _('Topic avatar'), class: 'avatar topic-avatar s60')
     - if @topic.name.length > max_topic_name_length
       %h1.gl-mt-3.str-truncated.has-tooltip{ title: @topic.name }