Rename ctx.Form() to ctx.FormString() and move code into own file (#16571)

Followup from #16562 prepare for #16567

* Rename ctx.Form() to ctx.FormString()
* Reimplement FormX func to need less code and cpu cycles
* Move code into own file

routers/api/v1/notify/threads.go

@@ -82,7 +82,7 @@ func ReadThread(ctx *context.APIContext) {
 		return
 	}
 
-	targetStatus := statusStringToNotificationStatus(ctx.Form("to-status"))
+	targetStatus := statusStringToNotificationStatus(ctx.FormString("to-status"))
 	if targetStatus == 0 {
 		targetStatus = models.NotificationStatusRead
 	}