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})}

js/forum/src/components/LogInModal.js

@@ -78,8 +78,7 @@ export default class LogInModal extends Modal {
 
         {app.forum.attribute('allowSignUp') ? (
           <p className="LogInModal-signUp">
-            {app.trans('core.forum.log_in_no_account_text')}
-            <a onclick={this.signUp.bind(this)}>{app.trans('core.forum.log_in_sign_up_link')}</a>
+            {app.trans('core.forum.log_in_sign_up_text', {a: <a onclick={this.signUp.bind(this)}/>})}
           </p>
         ) : ''}
       </div>

js/forum/src/components/SignUpModal.js

@@ -140,8 +140,7 @@ export default class SignUpModal extends Modal {
   footer() {
     return [
       <p className="SignUpModal-logIn">
-        {app.trans('core.forum.sign_up_already_have_account_text')}
-        <a onclick={this.logIn.bind(this)}>{app.trans('core.forum.sign_up_log_in_link')}</a>
+        {app.trans('core.forum.sign_up_log_in_text', {a: <a onclick={this.logIn.bind(this)}/>})}
       </p>
     ];
   }