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/pub.tmpl

@@ -4,7 +4,7 @@
 		<div class="ui form">
 			<div class="field">
 				<label>{{svg "octicon-terminal"}} {{ctx.Locale.Tr "packages.pub.install"}}</label>
-				<div class="markup"><pre class="code-block"><code>dart pub add {{.PackageDescriptor.Package.Name}}:{{.PackageDescriptor.Version.Version}} --hosted-url=<origin-url data-url="{{AppSubUrl}}/api/packages/{{.PackageDescriptor.Owner.Name}}/pub/"></origin-url></code></pre></div>
+				<div class="markup"><pre class="code-block"><code>dart pub add {{.PackageDescriptor.Package.Name}}:{{.PackageDescriptor.Version.Version}} --hosted-url={{ctx.AppFullLink}}/api/packages/{{.PackageDescriptor.Owner.Name}}/pub/</code></pre></div>
 			</div>
 			<div class="field">
 				<label>{{ctx.Locale.Tr "packages.registry.documentation" "Pub" "https://docs.gitea.com/usage/packages/pub/"}}</label>