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/admin/config.tmpl

@@ -11,10 +11,6 @@
 				<dd>{{AppVer}}{{.AppBuiltWith}}</dd>
 				<dt>{{ctx.Locale.Tr "admin.config.custom_conf"}}</dt>
 				<dd>{{.CustomConf}}</dd>
-				<dt>{{ctx.Locale.Tr "admin.config.app_url"}}</dt>
-				<dd>{{.AppUrl}}</dd>
-				<dt>{{ctx.Locale.Tr "admin.config.domain"}}</dt>
-				<dd>{{.Domain}}</dd>
 				<dt>{{ctx.Locale.Tr "admin.config.disable_router_log"}}</dt>
 				<dd>{{svg (Iif .DisableRouterLog "octicon-check" "octicon-x")}}</dd>