Audit Logs implementation (#1055)

* Copy audit logs impl from old branch and clean up

I suck at using git, so I'm gonna use brute force.

* Remove unnecessary TODOs

Category channels do not provide any new information, and the other
I forgot to remove beforehand

* Add invite update data, clean up after feedback

* Remove TODOs, add WebhookType enum for future use

WebhookType is a future-use type, as currently audit logs are the only
thing which may return it.

src/Discord.Net.Core/DiscordConfig.cs

@@ -4,10 +4,10 @@ namespace Discord
 {
     public class DiscordConfig
     {
-        public const int APIVersion = 6;        
+        public const int APIVersion = 6;
         public static string Version { get; } =
             typeof(DiscordConfig).GetTypeInfo().Assembly.GetCustomAttribute<AssemblyInformationalVersionAttribute>()?.InformationalVersion ??
-            typeof(DiscordConfig).GetTypeInfo().Assembly.GetName().Version.ToString(3) ?? 
+            typeof(DiscordConfig).GetTypeInfo().Assembly.GetName().Version.ToString(3) ??
             "Unknown";
 
         public static string UserAgent { get; } = $"DiscordBot (https://github.com/RogueException/Discord.Net, v{Version})";
@@ -20,10 +20,11 @@ namespace Discord
         public const int MaxMessagesPerBatch = 100;
         public const int MaxUsersPerBatch = 1000;
         public const int MaxGuildsPerBatch = 100;
+        public const int MaxAuditLogEntriesPerBatch = 100;
 
         /// <summary> Gets or sets how a request should act in the case of an error, by default. </summary>
         public RetryMode DefaultRetryMode { get; set; } = RetryMode.AlwaysRetry;
-        
+
         /// <summary> Gets or sets the minimum log level severity that will be sent to the Log event. </summary>
         public LogSeverity LogLevel { get; set; } = LogSeverity.Info;
 

src/Discord.Net.Core/Entities/AuditLogs/ActionType.cs

@@ -0,0 +1,50 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Discord
+{
+    /// <summary>
+    /// The action type within a <see cref="IAuditLogEntry"/>
+    /// </summary>
+    public enum ActionType
+    {
+        GuildUpdated = 1,
+
+        ChannelCreated = 10,
+        ChannelUpdated = 11,
+        ChannelDeleted = 12,
+
+        OverwriteCreated = 13,
+        OverwriteUpdated = 14,
+        OverwriteDeleted = 15,
+
+        Kick = 20,
+        Prune = 21,
+        Ban = 22,
+        Unban = 23,
+
+        MemberUpdated = 24,
+        MemberRoleUpdated = 25,
+
+        RoleCreated = 30,
+        RoleUpdated = 31,
+        RoleDeleted = 32,
+
+        InviteCreated = 40,
+        InviteUpdated = 41,
+        InviteDeleted = 42,
+
+        WebhookCreated = 50,
+        WebhookUpdated = 51,
+        WebhookDeleted = 52,
+
+        EmojiCreated = 60,
+        EmojiUpdated = 61,
+        EmojiDeleted = 62,
+
+        MessageDeleted = 72
+    }
+}

src/Discord.Net.Core/Entities/AuditLogs/IAuditLogData.cs

@@ -0,0 +1,14 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Discord
+{
+    /// <summary>
+    /// Represents data applied to an <see cref="IAuditLogEntry"/>
+    /// </summary>
+    public interface IAuditLogData
+    { }
+}

src/Discord.Net.Core/Entities/AuditLogs/IAuditLogEntry.cs

@@ -0,0 +1,34 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Discord
+{
+    /// <summary>
+    /// Represents an entry in an audit log
+    /// </summary>
+    public interface IAuditLogEntry : IEntity<ulong>
+    {
+        /// <summary>
+        /// The action which occured to create this entry
+        /// </summary>
+        ActionType Action { get; }
+
+        /// <summary>
+        /// The data for this entry. May be <see cref="null"/> if no data was available.
+        /// </summary>
+        IAuditLogData Data { get; }
+
+        /// <summary>
+        /// The user responsible for causing the changes
+        /// </summary>
+        IUser User { get; }
+
+        /// <summary>
+        /// The reason behind the change. May be <see cref="null"/> if no reason was provided.
+        /// </summary>
+        string Reason { get; }
+    }
+}

src/Discord.Net.Core/Entities/Guilds/IGuild.cs

@@ -139,6 +139,10 @@ namespace Discord
         /// <summary> Removes all users from this guild if they have not logged on in a provided number of days or, if simulate is true, returns the number of users that would be removed. </summary>
         Task<int> PruneUsersAsync(int days = 30, bool simulate = false, RequestOptions options = null);
 
+        /// <summary> Gets the specified number of audit log entries for this guild. </summary>
+        Task<IReadOnlyCollection<IAuditLogEntry>> GetAuditLogAsync(int limit = DiscordConfig.MaxAuditLogEntriesPerBatch,
+            CacheMode mode = CacheMode.AllowDownload, RequestOptions options = null);
+
         /// <summary> Gets the webhook in this guild with the provided id, or null if not found. </summary>
         Task<IWebhook> GetWebhookAsync(ulong id, RequestOptions options = null);
         /// <summary> Gets a collection of all webhooks for this guild. </summary>

src/Discord.Net.Core/Entities/Webhooks/WebhookType.cs

@@ -0,0 +1,14 @@
+namespace Discord
+{
+    /// <summary>
+    /// Represents the type of a webhook.
+    /// </summary>
+    /// <remarks>
+    /// This type is currently unused, and is only returned in audit log responses.
+    /// </remarks>
+    public enum WebhookType
+    {
+        /// <summary> An incoming webhook </summary>
+        Incoming = 1
+    }
+}