PLT-4507 Don't mount modals until opened to make resetting state automatic (#4358)

* Don't mount modals until opened to make resetting state automatic

* Move dimiss handler to be fired after modal exit animation complete

webapp/components/sidebar_right_menu.jsx

@@ -1,7 +1,7 @@
 // Copyright (c) 2015 Mattermost, Inc. All Rights Reserved.
 // See License.txt for license information.
 
-import AppDispatcher from '../dispatcher/app_dispatcher.jsx';
+import AppDispatcher from 'dispatcher/app_dispatcher.jsx';
 import TeamMembersModal from './team_members_modal.jsx';
 import ToggleModalButton from './toggle_modal_button.jsx';
 import UserSettingsModal from './user_settings/user_settings_modal.jsx';