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/nuget.tmpl

@@ -4,7 +4,7 @@
 		<div class="ui form">
 			<div class="field">
 				<label>{{svg "octicon-terminal"}} {{ctx.Locale.Tr "packages.nuget.registry"}}</label>
-				<div class="markup"><pre class="code-block"><code>dotnet nuget add source --name {{.PackageDescriptor.Owner.Name}} --username your_username --password your_token <origin-url data-url="{{AppSubUrl}}/api/packages/{{.PackageDescriptor.Owner.Name}}/nuget/index.json"></origin-url></code></pre></div>
+				<div class="markup"><pre class="code-block"><code>dotnet nuget add source --name {{.PackageDescriptor.Owner.Name}} --username your_username --password your_token {{ctx.AppFullLink}}/api/packages/{{.PackageDescriptor.Owner.Name}}/nuget/index.json</code></pre></div>
 			</div>
 			<div class="field">
 				<label>{{svg "octicon-terminal"}} {{ctx.Locale.Tr "packages.nuget.install"}}</label>