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

templates/package/content/composer.tmpl

@@ -7,7 +7,7 @@
 				<div class="markup"><pre class="code-block"><code>{
 	"repositories": [{
 			"type": "composer",
-			"url": "<origin-url data-url="{{AppSubUrl}}/api/packages/{{.PackageDescriptor.Owner.Name}}/composer"></origin-url>"
+			"url": "{{ctx.AppFullLink}}/api/packages/{{.PackageDescriptor.Owner.Name}}/composer"
 		}
 	]
 }</code></pre></div>