Paginate reactions - solved #1007 (#1022)

* Cleaned up and refactored slightly

* Resolves #971

* Adds support for default avatars and resolves #971

* Amendment

* Final amendment

* Paginating reactions

* Amendments based on feedback

* Further amendment based on review

* Final(?) amendment

* Removes default limit and after user id

* Removes fromUserId; cleans up model creation; replaces function with individual parameters

src/Discord.Net.Core/DiscordConfig.cs

@@ -1,4 +1,4 @@
-using System.Reflection;
+using System.Reflection;
 
 namespace Discord
 {
@@ -20,6 +20,7 @@ namespace Discord
         public const int MaxMessagesPerBatch = 100;
         public const int MaxUsersPerBatch = 1000;
         public const int MaxGuildsPerBatch = 100;
+        public const int MaxUserReactionsPerBatch = 100;
         public const int MaxAuditLogEntriesPerBatch = 100;
 
         /// <summary> Gets or sets how a request should act in the case of an error, by default. </summary>

src/Discord.Net.Core/Entities/Messages/IUserMessage.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections.Generic;
 using System.Threading.Tasks;
 
@@ -23,7 +23,7 @@ namespace Discord
         /// <summary> Removes all reactions from this message. </summary>
         Task RemoveAllReactionsAsync(RequestOptions options = null);
         /// <summary>  Gets all users that reacted to a message with a given emote </summary>
-        Task<IReadOnlyCollection<IUser>> GetReactionUsersAsync(IEmote emoji, int limit = 100, ulong? afterUserId = null, RequestOptions options = null);
+        IAsyncEnumerable<IReadOnlyCollection<IUser>> GetReactionUsersAsync(IEmote emoji, int limit, RequestOptions options = null);
 
         /// <summary> Transforms this message's text into a human readable form by resolving its tags. </summary>
         string Resolve(