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.

modules/structs/repo.go

@@ -63,6 +63,7 @@ type Repository struct {
 	Language      string      `json:"language"`
 	LanguagesURL  string      `json:"languages_url"`
 	HTMLURL       string      `json:"html_url"`
+	Link          string      `json:"link"`
 	SSHURL        string      `json:"ssh_url"`
 	CloneURL      string      `json:"clone_url"`
 	OriginalURL   string      `json:"original_url"`