Refactor locale&string&template related code (#29165)

Clarify when "string" should be used (and be escaped), and when
"template.HTML" should be used (no need to escape)

And help PRs like  #29059 , to render the error messages correctly.

services/forms/repo_form.go

@@ -314,7 +314,7 @@ func (f *NewSlackHookForm) Validate(req *http.Request, errs binding.Errors) bind
 		errs = append(errs, binding.Error{
 			FieldNames:     []string{"Channel"},
 			Classification: "",
-			Message:        ctx.Tr("repo.settings.add_webhook.invalid_channel_name"),
+			Message:        ctx.Locale.TrString("repo.settings.add_webhook.invalid_channel_name"),
 		})
 	}
 	return middleware.Validate(errs, ctx.Data, f, ctx.Locale)