Mostly backend models stuff

Changed some models, did some work on database, created a class diagram.

KTUSAPS.Data/Model/Vote.cs

@@ -0,0 +1,18 @@
+using System;
+using System.Collections.Generic;
+using System.ComponentModel.DataAnnotations;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace KTUSAPS.Data.Model
+{
+    public class Vote
+    {
+        [MaxLength(64)]
+        public string UserId { get; set; }
+        public int ProblemId { get; set; }
+        public PublishedProblem Problem { get; set; }
+
+    }
+}