UX: i18n text for topic list column headers

assets/javascripts/discourse/templates/components/knowledge-explorer-topic-list.hbs

@@ -2,7 +2,7 @@
   <table class="topic-list">
     <thead>
       <th {{action "sortListTitle"}}>
-        Topic
+        {{i18n 'knowledge_explorer.column_titles.topic'}}
         {{#if sortTitle}}
           {{#if ascending}}
             {{d-icon 'angle-up'}}
@@ -12,7 +12,7 @@
         {{/if}}
       </th>
       <th {{action "sortListActivity"}}>
-        Activity
+        {{i18n 'knowledge_explorer.column_titles.activity'}}
         {{#if sortActivity}}
           {{#if ascending}}
             {{d-icon 'angle-up'}}

config/locales/client.en.yml

@@ -2,6 +2,9 @@ en:
   js:
     knowledge_explorer:
       title: "Knowledge Explorer"
+      column_titles:
+        topic: "Topic"
+        activity: "Activity"
       no_topics: "No topics in Knowledge Explorer."
       categories: "Categories"
       tags: "Tags"