Clean up AppURL, remove legacy origin-url webcomponent (#37090)

1. `origin-url` was introduced in the past when there was no good
framework support to detect current host url
    * It is not needed anymore
    * Removing it makes the code clearer
2. Separate template helper functions for different templates (web
page/mail)
3. The "AppURL" info is removed from admin config page: it doesn't
really help.
    * We already have various app url checks at many places

web_src/js/webcomponents/index.ts

@@ -1,6 +1,5 @@
 import './polyfills.ts';
 import './relative-time.ts';
-import './origin-url.ts';
 import './overflow-menu.ts';
 import {isDarkTheme} from '../utils.ts';