api: add slow mode

This adds the following property to ITextChannel
- SlowMode (int)

The SlowMode field defines the time in seconds users must wait between
sending messages; if zero, slow-mode is disabled.

Bots are not affected by slow-mode, and as such, no changes need to be
made to the REST client's internal ratelimiting. This is strictly a
read/write cosmetic property for bots.

src/Discord.Net.Rest/API/Common/Channel.cs

@@ -1,4 +1,4 @@
-#pragma warning disable CS1591
+#pragma warning disable CS1591
 using Newtonsoft.Json;
 using System;
 
@@ -33,6 +33,8 @@ namespace Discord.API
         public Optional<DateTimeOffset?> LastPinTimestamp { get; set; }
         [JsonProperty("nsfw")]
         public Optional<bool> Nsfw { get; set; }
+        [JsonProperty("rate_limit_per_user")]
+        public Optional<int> SlowMode { get; set; }
 
         //VoiceChannel
         [JsonProperty("bitrate")]

src/Discord.Net.Rest/API/Rest/ModifyTextChannelParams.cs

@@ -1,4 +1,4 @@
-#pragma warning disable CS1591
+#pragma warning disable CS1591
 using Newtonsoft.Json;
 
 namespace Discord.API.Rest
@@ -10,5 +10,7 @@ namespace Discord.API.Rest
         public Optional<string> Topic { get; set; }
         [JsonProperty("nsfw")]
         public Optional<bool> IsNsfw { get; set; }
+        [JsonProperty("rate_limit_per_user")]
+        public Optional<int> SlowMode { get; set; }
     }
 }