Added .banprune command which sets how many days worth of messages will be pruned when bot (soft)bans a person either through a command or another punishment feature. Also updated .next usage string.

src/NadekoBot/Db/NadekoContext.cs

@@ -330,6 +330,10 @@ public abstract class NadekoContext : DbContext
         #region BanTemplate
 
         modelBuilder.Entity<BanTemplate>().HasIndex(x => x.GuildId).IsUnique();
+        modelBuilder.Entity<BanTemplate>()
+            .Property(x => x.PruneDays)
+            .HasDefaultValue(null)
+            .IsRequired(false);
 
         #endregion