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_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}},