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.WebSocket/Entities/Guilds/SocketGuild.cs

@@ -438,6 +438,10 @@ namespace Discord.WebSocket
             _downloaderPromise.TrySetResultAsync(true);
         }
 
+        //Audit logs
+        public IAsyncEnumerable<IReadOnlyCollection<RestAuditLogEntry>> GetAuditLogsAsync(int limit, RequestOptions options = null)
+            => GuildHelper.GetAuditLogsAsync(this, Discord, null, limit, options);
+
         //Webhooks
         public Task<RestWebhook> GetWebhookAsync(ulong id, RequestOptions options = null)
             => GuildHelper.GetWebhookAsync(this, Discord, id, options);
@@ -703,6 +707,14 @@ namespace Discord.WebSocket
         Task<IGuildUser> IGuild.GetOwnerAsync(CacheMode mode, RequestOptions options)
             => Task.FromResult<IGuildUser>(Owner);
 
+        async Task<IReadOnlyCollection<IAuditLogEntry>> IGuild.GetAuditLogAsync(int limit, CacheMode cacheMode, RequestOptions options)
+        {
+            if (cacheMode == CacheMode.AllowDownload)
+                return (await GetAuditLogsAsync(limit, options).FlattenAsync().ConfigureAwait(false)).ToImmutableArray();
+            else
+                return ImmutableArray.Create<IAuditLogEntry>();
+        }
+
         async Task<IWebhook> IGuild.GetWebhookAsync(ulong id, RequestOptions options)
             => await GetWebhookAsync(id, options);
         async Task<IReadOnlyCollection<IWebhook>> IGuild.GetWebhooksAsync(RequestOptions options)