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

vite.config.ts

@@ -32,7 +32,6 @@ for (const path of globSync('web_src/css/themes/*.css', {cwd: import.meta.dirnam
 const webComponents = new Set([
   // our own, in web_src/js/webcomponents
   'overflow-menu',
-  'origin-url',
   'relative-time',
   // from dependencies
   'markdown-toolbar',