Don't use `date-separator` CSS class for the new messages indicator

sass/_messages.scss

@@ -11,7 +11,8 @@
         .mention--self {
             font-weight: normal;
         }
-        &.date-separator {
+        &.date-separator,
+        &.separator {
             height: 2em;
             margin: 0;
             position: relative;

src/templates/chat_message.js

@@ -6,7 +6,7 @@ import { renderAvatar } from './../templates/directives/avatar';
 export default (o) => {
     const i18n_new_messages = __('New messages');
     return html`
-        ${ o.is_first_unread ? html`<div class="message date-separator"><hr class="separator"><span class="separator-text">${ i18n_new_messages }</span></div>` : '' }
+        ${ o.is_first_unread ? html`<div class="message separator"><hr class="separator"><span class="separator-text">${ i18n_new_messages }</span></div>` : '' }
         <div class="message chat-msg ${ o.getExtraMessageClasses() }"
                 data-isodate="${o.time}"
                 data-msgid="${o.msgid}"