Extract leftover core strings

Adds app.trans calls for a couple strings in core:
- The "there are no discussions" message in DiscussionList.js
- The user deletion confirmation message in UserControls.js
- Also adds new HTML-style tags to LogInModal.js and SignUpModal.js

js/forum/src/components/DiscussionList.js

@@ -55,7 +55,7 @@ export default class DiscussionList extends Component {
     }
 
     if (this.discussions.length === 0 && !this.loading) {
-      const text = 'Looks like there are no discussions here. Why don\'t you create a new one?';
+      const text = app.trans('core.admin.discussion_list_empty_text');
       return (
         <div className="DiscussionList">
           {Placeholder.component({text})}