Fix/Implement various invite-related behaviors (#1023)

* Initial support for invite member count arg

* Fix IDiscordClient#GetInviteAsync behavior

- Previously, the GetInviteAsync method would return null since it couldn't be parsed as a simple RestInvite object. The object should be a RestInviteMetadata instead.

* Fix methods that didn't comply with the interface

* Change with_counts REST behaviour

* Remove unnecessary JSON prop

* Remove AcceptAsync

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

@@ -1,4 +1,4 @@
-#pragma warning disable CS1591
+#pragma warning disable CS1591
 using Newtonsoft.Json;
 
 namespace Discord.API
@@ -11,5 +11,9 @@ namespace Discord.API
         public InviteGuild Guild { get; set; }
         [JsonProperty("channel")]
         public InviteChannel Channel { get; set; }
+        [JsonProperty("approximate_presence_count")]
+        public Optional<int?> PresenceCount { get; set; }
+        [JsonProperty("approximate_member_count")]
+        public Optional<int?> MemberCount { get; set; }
     }
 }