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/arch.tmpl

@@ -7,7 +7,7 @@
 				<div class="markup"><pre class="code-block"><code>{{range $i, $repo := .Repositories}}{{if $i}}
 {{end}}[{{$repo}}]
 SigLevel = Optional TrustAll
-Server = <origin-url data-url="{{AppSubUrl}}/api/packages/{{$.PackageDescriptor.Owner.Name}}/arch/$repo/$arch"></origin-url>
+Server = {{ctx.AppFullLink}}/api/packages/{{$.PackageDescriptor.Owner.Name}}/arch/$repo/$arch
 {{end}}</code></pre></div>
 			</div>
 			<div class="field">