[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/Channels/ChannelHelper.cs

@@ -38,6 +38,7 @@ namespace Discord.Rest
                         Deny = overwrite.Permissions.DenyValue.ToString()
                     }).ToArray()
                     : Optional.Create<API.Overwrite[]>(),
+                Flags = args.Flags.GetValueOrDefault(),
             };
             return await client.ApiClient.ModifyGuildChannelAsync(channel.Id, apiArgs, options).ConfigureAwait(false);
         }