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_watch.go

@@ -52,5 +52,5 @@ func IssueWatch(ctx *context.Context) {
 		return
 	}
 
-	ctx.Redirect(issue.HTMLURL())
+	ctx.Redirect(issue.Link())
 }