Check that messages older than two weeks are not passed to bulk delete

This resolves #477

src/Discord.Net.Core/Utils/DateTimeUtils.cs

@@ -12,6 +12,8 @@ namespace Discord
 
         public static DateTimeOffset FromSnowflake(ulong value)
             => FromUnixMilliseconds((long)((value >> 22) + 1420070400000UL));
+        public static ulong ToSnowflake(DateTimeOffset value)
+            => (ulong)(ToUnixMilliseconds(value) - 1420070400000L) << 22;
 
         public static DateTimeOffset FromTicks(long ticks)
             => new DateTimeOffset(ticks, TimeSpan.Zero);