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.Core/Entities/Invites/IInvite.cs

@@ -1,4 +1,4 @@
-using System.Threading.Tasks;
+using System.Threading.Tasks;
 
 namespace Discord
 {
@@ -22,8 +22,9 @@ namespace Discord
         ulong GuildId { get; }
         /// <summary> Gets the name of the guild this invite is linked to. </summary>
         string GuildName { get; }
-
-        /// <summary> Accepts this invite and joins the target guild. This will fail on bot accounts. </summary>
-        Task AcceptAsync(RequestOptions options = null);
+        /// <summary> Gets the approximated count of online members in the guild. </summary>
+        int? PresenceCount { get; }
+        /// <summary> Gets the approximated count of total members in the guild. </summary>
+        int? MemberCount { get; }
     }
 }

src/Discord.Net.Core/IDiscordClient.cs

@@ -27,7 +27,7 @@ namespace Discord
         Task<IReadOnlyCollection<IGuild>> GetGuildsAsync(CacheMode mode = CacheMode.AllowDownload, RequestOptions options = null);
         Task<IGuild> CreateGuildAsync(string name, IVoiceRegion region, Stream jpegIcon = null, RequestOptions options = null);
         
-        Task<IInvite> GetInviteAsync(string inviteId, RequestOptions options = null);
+        Task<IInvite> GetInviteAsync(string inviteId, bool withCount = false, RequestOptions options = null);
 
         Task<IUser> GetUserAsync(ulong id, CacheMode mode = CacheMode.AllowDownload, RequestOptions options = null);
         Task<IUser> GetUserAsync(string username, string discriminator, RequestOptions options = null);