Fix comments header styles applying to comments

Previous way was a bad way of doing the styles and some of the rules
were leaking through into the comments themselves. This should be more
specific.

tildes/scss/modules/_topic.scss

@@ -245,8 +245,7 @@
   overflow: auto;
 }
 
-.topic-comments {
-  header {
+.topic-comments-header {
   display: flex;
   flex-wrap: wrap;
   align-items: center;
@@ -262,7 +261,6 @@
   .form-listing-options {
     margin-left: auto;
   }
-  }
 }
 
 .is-topic-mine {

tildes/tildes/templates/topic.jinja2

@@ -144,7 +144,7 @@
 
 {% if topic.num_comments > 0 %}
   <section class="topic-comments">
-    <header>
+    <header class="topic-comments-header">
       <h2>
       {% trans num_comments=topic.num_comments %}
       {{ num_comments }} comment