[Feature] Add missing properties in forum & thread channels (#2469)

* add `AppliedTags` property

* convert collections into immutable arrays

* remove "not supported" remark

* implement `ThreadChannelProperties`

* Add `DefaultSlowModeInterval` and `DefaultSlowModeInterval` properties to forum channels

* add `Moderated` property to `ForumTag``

* `ForumTag` inherits `ISnowflakeEntity`

* Fix `DiscordRestClient.GetChannelAsync` not getting forum channel

* a lot of changes

added:
- channel flags
- `ForumTagBuilder`
- imroved channel modification

* fixed a bug in forum tag emoji parsing

* inherit forum channel from `INesteeChannel`

* implement `INestedChannel` in forum channels

* Add `Flags` property to channels

* add iteraface for forum tags & add equality operators

* Add default reaction emoji property

* add support for modifing default reaction & some renaming

* add createForumChannelAsync to guild

* *fix resharper being a d... and moving code to next line*

* add a `ForumChannels` property

* Some fixes & add support for `default_sort_order`

* fix misleading comment

* fix #2502

* support creating post with applied tags

* fix xmldoc

* set category id on model update

* add limit checks for tag count

src/Discord.Net.Rest/Entities/Guilds/GuildHelper.cs

@@ -1,3 +1,4 @@
+using Discord.API;
 using Discord.API.Rest;
 using System;
 using System.Collections.Generic;
@@ -252,6 +253,7 @@ namespace Discord.Rest
                         Deny = overwrite.Permissions.DenyValue.ToString()
                     }).ToArray()
                     : Optional.Create<API.Overwrite[]>(),
+                DefaultAutoArchiveDuration = props.AutoArchiveDuration
             };
             var model = await client.ApiClient.CreateGuildChannelAsync(guild.Id, args, options).ConfigureAwait(false);
             return RestTextChannel.Create(client, guild, model);
@@ -338,6 +340,65 @@ namespace Discord.Rest
             var model = await client.ApiClient.CreateGuildChannelAsync(guild.Id, args, options).ConfigureAwait(false);
             return RestCategoryChannel.Create(client, guild, model);
         }
+
+        /// <exception cref="ArgumentNullException"><paramref name="name"/> is <c>null</c>.</exception>
+        public static async Task<RestForumChannel> CreateForumChannelAsync(IGuild guild, BaseDiscordClient client,
+            string name, RequestOptions options, Action<ForumChannelProperties> func = null)
+        {
+            if (name == null)
+                throw new ArgumentNullException(paramName: nameof(name));
+
+            var props = new ForumChannelProperties();
+            func?.Invoke(props);
+
+            Preconditions.AtMost(props.Tags.IsSpecified ? props.Tags.Value.Count() : 0, 5, nameof(props.Tags), "Forum channel can have max 20 tags.");
+
+            var args = new CreateGuildChannelParams(name, ChannelType.Forum)
+            {
+                Position = props.Position,
+                Overwrites = props.PermissionOverwrites.IsSpecified
+                    ? props.PermissionOverwrites.Value.Select(overwrite => new API.Overwrite
+                    {
+                        TargetId = overwrite.TargetId,
+                        TargetType = overwrite.TargetType,
+                        Allow = overwrite.Permissions.AllowValue.ToString(),
+                        Deny = overwrite.Permissions.DenyValue.ToString()
+                    }).ToArray()
+                    : Optional.Create<API.Overwrite[]>(),
+                SlowModeInterval = props.ThreadCreationInterval,
+                AvailableTags = props.Tags.GetValueOrDefault(Array.Empty<ForumTagProperties>()).Select(
+                    x => new ModifyForumTagParams
+                    {
+                        Id = x.Id,
+                        Name = x.Name,
+                        EmojiId = x.Emoji is Emote emote
+                            ? emote.Id
+                            : Optional<ulong?>.Unspecified,
+                        EmojiName = x.Emoji is Emoji emoji
+                            ? emoji.Name
+                            : Optional<string>.Unspecified,
+                        Moderated = x.IsModerated
+                    }).ToArray(),
+                DefaultReactionEmoji = props.DefaultReactionEmoji.IsSpecified
+                    ? new API.ModifyForumReactionEmojiParams
+                    {
+                        EmojiId = props.DefaultReactionEmoji.Value is Emote emote ?
+                            emote.Id : Optional<ulong?>.Unspecified,
+                        EmojiName = props.DefaultReactionEmoji.Value is Emoji emoji ?
+                            emoji.Name : Optional<string>.Unspecified
+                    }
+                    : Optional<ModifyForumReactionEmojiParams>.Unspecified,
+                ThreadRateLimitPerUser = props.DefaultSlowModeInterval,
+                CategoryId = props.CategoryId,
+                IsNsfw = props.IsNsfw,
+                Topic = props.Topic,
+                DefaultAutoArchiveDuration = props.AutoArchiveDuration,
+                DefaultSortOrder = props.DefaultSortOrder.GetValueOrDefault(ForumSortOrder.LatestActivity)
+            };
+
+            var model = await client.ApiClient.CreateGuildChannelAsync(guild.Id, args, options).ConfigureAwait(false);
+            return RestForumChannel.Create(client, guild, model);
+        }
         #endregion
 
         #region Voice Regions