Use link in UI which returned a relative url but not html_url which contains an absolute url (#21986)

partially fix #19345

This PR add some `Link` methods for different objects. The `Link`
methods are not different from `HTMLURL`, they are lack of the absolute
URL. And most of UI `HTMLURL` have been replaced to `Link` so that users
can visit them from a different domain or IP.

This PR also introduces a new javascript configuration
`window.config.reqAppUrl` which is different from `appUrl` which is
still an absolute url but the domain has been replaced to the current
requested domain.

web_src/js/test/setup.js

@@ -2,4 +2,5 @@ window.config = {
   csrfToken: 'test-csrf-token-123456',
   pageData: {},
   i18n: {},
+  appSubUrl: '',
 };