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.

(cherry picked from commit f3eb835886031df7a562abc123c3f6011c81eca8)

Conflicts:
	modules/web/middleware/binding.go
	routers/web/feed/convert.go
	tests/integration/branches_test.go
	tests/integration/repo_branch_test.go
	trivial context conflicts

routers/web/auth/password.go

@@ -37,7 +37,7 @@ func ForgotPasswd(ctx *context.Context) {
 	ctx.Data["Title"] = ctx.Tr("auth.forgot_password_title")
 
 	if setting.MailService == nil {
-		log.Warn(ctx.Tr("auth.disable_forgot_password_mail_admin"))
+		log.Warn("no mail service configured")
 		ctx.Data["IsResetDisable"] = true
 		ctx.HTML(http.StatusOK, tplForgotPassword)
 		return