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/Models/BanTemplate.cs

@@ -5,4 +5,5 @@ public class BanTemplate : DbEntity
 {
     public ulong GuildId { get; set; }
     public string Text { get; set; }
+    public int? PruneDays { get; set; }
 }