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/Migrations/Sqlite/NadekoSqliteContextModelSnapshot.cs

@@ -2399,6 +2399,20 @@ namespace NadekoBot.Migrations
                     b.ToTable("GreetSettings");
                 });
 
+            modelBuilder.Entity("NadekoBot.Services.Rakeback", b =>
+                {
+                    b.Property<ulong>("UserId")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("INTEGER");
+
+                    b.Property<decimal>("Amount")
+                        .HasColumnType("TEXT");
+
+                    b.HasKey("UserId");
+
+                    b.ToTable("Rakeback");
+                });
+
             modelBuilder.Entity("NadekoBot.Services.UserBetStats", b =>
                 {
                     b.Property<int>("Id")