replaced string `discussion` with string `thread` in thread header

app/assets/javascripts/notes/components/note_header.vue

@@ -70,7 +70,7 @@ export default {
         @click="handleToggle"
       >
         <i :class="toggleChevronClass" class="fa" aria-hidden="true"></i>
-        {{ __('Toggle discussion') }}
+        {{ __('Toggle thread') }}
       </button>
     </div>
     <a

app/views/discussions/_discussion.html.haml

@@ -13,7 +13,7 @@
                 = icon("chevron-up")
               - else
                 = icon("chevron-down")
-              = _('Toggle discussion')
+              = _('Toggle thread')
           = link_to_member(@project, discussion.author, avatar: false)
 
           .inline.discussion-headline-light