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/base/head_opengraph.tmpl

@@ -16,7 +16,7 @@
 		{{end}}
 	{{else if or .PageIsDiff .IsViewFile}}
 		<meta property="og:title" content="{{.Title}}">
-		<meta property="og:url" content="{{AppUrl}}{{.Link}}">
+		<meta property="og:url" content="{{ctx.AppFullLink $.Link}}">
 		{{if and .PageIsDiff .Commit}}
 			{{- $commitMessageParts := StringUtils.Cut .Commit.Message "\n" -}}
 			{{- $commitMessageBody := index $commitMessageParts 1 -}}
@@ -41,7 +41,7 @@
 	<meta property="og:title" content="{{AppName}}">
 	<meta property="og:type" content="website">
 	<meta property="og:image" content="{{AssetUrlPrefix}}/img/logo.png">
-	<meta property="og:url" content="{{AppUrl}}">
+	<meta property="og:url" content="{{ctx.AppFullLink}}">
 	<meta property="og:description" content="{{MetaDescription}}">
 {{end}}
 <meta property="og:site_name" content="{{AppName}}">

templates/base/head_script.tmpl

@@ -7,7 +7,7 @@ If you introduce mistakes in it, Gitea JavaScript code wouldn't run correctly.
 	window.addEventListener('error', function(e) {window._globalHandlerErrors=window._globalHandlerErrors||[]; window._globalHandlerErrors.push(e);});
 	window.addEventListener('unhandledrejection', function(e) {window._globalHandlerErrors=window._globalHandlerErrors||[]; window._globalHandlerErrors.push(e);});
 	window.config = {
-		appUrl: '{{AppUrl}}',
+		appUrl: '{{ctx.AppFullLink "/"}}',
 		appSubUrl: '{{AppSubUrl}}',
 		assetUrlPrefix: '{{AssetUrlPrefix}}',
 		runModeIsProd: {{.RunModeIsProd}},