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/devtest/gitea-ui.tmpl

@@ -84,12 +84,6 @@
 		</div>
 	</div>
 
-	<div>
-		<h1>&lt;origin-url&gt;</h1>
-		<div><origin-url data-url="test/url"></origin-url></div>
-		<div><origin-url data-url="/test/url"></origin-url></div>
-	</div>
-
 	<div>
 		<h1>&lt;overflow-menu&gt;</h1>
 		<overflow-menu class="ui secondary pointing tabular borderless menu">