popovers: Add newline between checkboxes in Move topic menu.

This is cleaner UI and avoids the spacing looking weird if both fit on a line at
a given zoom level.

Fixes #19875.

static/templates/move_topic_to_stream.hbs

@@ -28,6 +28,7 @@
                         <span></span>
                         {{t "Send notification to new topic" }}
                     </label>
+                    <br/>
                     <label class="checkbox">
                         <input class="send_notification_to_old_thread" name="send_notification_to_old_thread" type="checkbox" {{#if notify_old_thread}}checked="checked"{{/if}} />
                         <span></span>