Fix improper HTMLURL usages in Go code (#22839)

In Go code, HTMLURL should be only used for external systems, like
API/webhook/mail/notification, etc.

If a URL is used by `Redirect` or rendered in a template, it should be a
relative URL (aka `Link()` in Gitea)

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

routers/web/repo/issue_stopwatch.go

@@ -40,7 +40,7 @@ func IssueStopwatch(c *context.Context) {
 		c.Flash.Success(c.Tr("repo.issues.tracker_auto_close"))
 	}
 
-	url := issue.HTMLURL()
+	url := issue.Link()
 	c.Redirect(url, http.StatusSeeOther)
 }
 
@@ -72,7 +72,7 @@ func CancelStopwatch(c *context.Context) {
 		})
 	}
 
-	url := issue.HTMLURL()
+	url := issue.Link()
 	c.Redirect(url, http.StatusSeeOther)
 }