Extract leftover strings

Extracts strings that were missed previously in:
- Dashboard page of admin interface.
- Edit Custom CSS modal of admin interface.
- Settings modal of admin interface.
- Post activity list on user page of forum UI.
Hopefully there aren't any more!

js/forum/src/components/PostsUserPage.js

@@ -65,7 +65,7 @@ export default class PostsUserPage extends UserPage {
           {this.posts.map(post => (
             <li>
               <div className="PostsUserPage-discussion">
-                In <a href={app.route.post(post)} config={m.route}>{post.discussion().title()}</a>
+                {app.translator.trans('core.forum.user.in_discussion_text', {discussion: <a href={app.route.post(post)} config={m.route}>{post.discussion().title()}</a>})}
               </div>
               {CommentPost.component({post, showDiscussionTitle: true})}
             </li>