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

routers/web/admin/config.go

@@ -123,9 +123,7 @@ func Config(ctx *context.Context) {
 	ctx.Data["PageIsAdminConfigSummary"] = true
 
 	ctx.Data["CustomConf"] = setting.CustomConf
-	ctx.Data["AppUrl"] = setting.AppURL
 	ctx.Data["AppBuiltWith"] = setting.AppBuiltWith
-	ctx.Data["Domain"] = setting.Domain
 	ctx.Data["RunUser"] = setting.RunUser
 	ctx.Data["RunMode"] = util.ToTitleCase(setting.RunMode)
 	ctx.Data["GitVersion"] = git.DefaultFeatures().VersionInfo()