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.

routers/web/repo/repo.go

@@ -569,6 +569,7 @@ func SearchRepo(ctx *context.Context) {
 			Mirror:   repo.IsMirror,
 			Stars:    repo.NumStars,
 			HTMLURL:  repo.HTMLURL(),
+			Link:     repo.Link(),
 			Internal: !repo.IsPrivate && repo.Owner.Visibility == api.VisibleTypePrivate,
 		}
 	}