Add dark mode toggle in mobile view

Fixes #2534

Signed-off-by: David Mehren <git@herrmehren.de>

public/views/hedgedoc/header.ejs

@@ -70,6 +70,11 @@
                 <li role="presentation"><a role="menuitem" class="ui-help" href="#" data-toggle="modal" data-target=".help-modal"><i class="fa fa-question-circle fa-fw"></i> Help</a>
                 </li>
             </ul>
+            <div class="btn-group" data-toggle="buttons">
+                <label class="btn ui-night" title="<%= __('Night Theme') %>">
+                    <input type="checkbox" name="night"><i class="fa fa-moon-o"></i>
+                </label>
+            </div>
             <a class="btn btn-link ui-mode">
                 <i class="fa fa-pencil"></i>
             </a>