Fix more HTMLURL in templates (#22831)

I haven't tested `runs_list.tmpl` but I think it could be right.

After this PR, besides the `<meta .. HTMLURL>` in html head, the only
explicit HTMLURL usage is in `pull_merge_instruction.tmpl`, which
doesn't affect users too much and it's difficult to fix at the moment.

There are still many usages of `AppUrl` in the templates (eg: the
package help manual), they are similar problems as the HTMLURL in
pull_merge_instruction, and they might be fixed together in the future.

Diff without space:
https://github.com/go-gitea/gitea/pull/22831/files?diff=unified&w=1

modules/templates/helper.go

@@ -72,6 +72,10 @@ func NewFuncMap() []template.FuncMap {
 			return setting.StaticURLPrefix + "/assets"
 		},
 		"AppUrl": func() string {
+			// The usage of AppUrl should be avoided as much as possible,
+			// because the AppURL(ROOT_URL) may not match user's visiting site and the ROOT_URL in app.ini may be incorrect.
+			// And it's difficult for Gitea to guess absolute URL correctly with zero configuration,
+			// because Gitea doesn't know whether the scheme is HTTP or HTTPS unless the reverse proxy could tell Gitea.
 			return setting.AppURL
 		},
 		"AppVer": func() string {