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/generic.tmpl

@@ -6,7 +6,7 @@
 				<label>{{svg "octicon-terminal"}} {{ctx.Locale.Tr "packages.generic.download"}}</label>
 				<div class="markup"><pre class="code-block"><code>
 {{- range .PackageDescriptor.Files -}}
-curl -OJ <origin-url data-url="{{AppSubUrl}}/api/packages/{{$.PackageDescriptor.Owner.Name}}/generic/{{$.PackageDescriptor.Package.Name}}/{{$.PackageDescriptor.Version.Version}}/{{.File.Name}}"></origin-url>
+curl -OJ {{ctx.AppFullLink}}/api/packages/{{$.PackageDescriptor.Owner.Name}}/generic/{{$.PackageDescriptor.Package.Name}}/{{$.PackageDescriptor.Version.Version}}/{{.File.Name}}
 {{end -}}
 				</code></pre></div>
 			</div>