Update Guild and Message Models (#1165)

* Add ExplicitContentFilter property to Guild

* re-order properties to match order listed on api docs

* re-order SystemChannelId to match api docs

* Implement ApplicationId in Guild model

* Add ExplicitContentFilter property to Guild

* re-order properties to match order listed on api docs

* re-order SystemChannelId to match api docs

* Implement ApplicationId in Guild model

* Improve xmldoc for IGuild ExplicitContentFilter

* Update xmldoc

* docs "Id" -> "ID"

* rename Test.GuildPermissions to a more general Test.Guilds

* Add ExplicitContentFilter to GuildProperties

* Add a test for ExplicitContentFilterLevel modification behavior

* Implement ModifyAsync behavior

* simplify ExplicitContentFilter test

* Add RestGuild ApplicationId inheritdoc

* Implement message Activity and Application model update

* RestMessage Application and Activity implementation

* add ToString to MessageApplication

* Add IconUrl property to MessageApplication

* clean up whitespace

* another excessive whitespace removal

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

@@ -44,5 +44,11 @@ namespace Discord.API
         public Optional<bool> Pinned { get; set; }
         [JsonProperty("reactions")]
         public Optional<Reaction[]> Reactions { get; set; }
+        // sent with Rich Presence-related chat embeds
+        [JsonProperty("activity")]
+        public Optional<MessageActivity> Activity { get; set; }
+        // sent with Rich Presence-related chat embeds
+        [JsonProperty("application")]
+        public Optional<MessageApplication> Application { get; set; }
     }
 }