Use link in UI which returned a relative url but not html_url which contains an absol...

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.