Move admin/themes to src/Module

- Add Module\Admin\Themes\Index class
- Add route for admin/themes
- Add themes admin aside menu entry
- Add admin/addons/index.tpl template
- Remove theme list from mod/admin

src/Module/BaseAdminModule.php

@@ -53,6 +53,7 @@ abstract class BaseAdminModule extends BaseModule
 				'federation'   => ['admin/federation'  , L10n::t('Federation Statistics')   , 'federation']
 			]],
 			'configuration' => [L10n::t('Configuration'), [
+				'themes'       => ['admin/themes'      , L10n::t('Themes')                  , 'themes'],
 				'tos'          => ['admin/tos'         , L10n::t('Terms of Service')        , 'tos'],
 			]],
 		];