add: added .rakeback to get a part of the house edge back. Rakeback is accumulated by betting (not winning or losing in particular). All games have manually specified rakeback values

add: slot now has 1 more icon (wheat!), and multipliers have been modified to even out the gains
change: betroll is improved (around 2% better payout), as 66 is now a winning number, not a losing one

src/NadekoBot/Db/NadekoContext.cs

@@ -74,6 +74,13 @@ public abstract class NadekoContext : DbContext
 
     protected override void OnModelCreating(ModelBuilder modelBuilder)
     {
+        #region Rakeback
+
+        modelBuilder.Entity<Rakeback>()
+                    .HasKey(x => x.UserId);
+        
+        #endregion
+        
         #region UserBetStats
         
         modelBuilder.Entity<UserBetStats>()