Add missing forum fields to guild channel

[Chronophylos]

Added flags, total_messages_sent, available_tags, applied_tags, default_reaction_emoji, default_thread_rate_limit_per_user and default_sort_order fields to GuildChannel.
Added ForumTag, ForumTagId, DefaultReaction, and SortOrder objects.
Boxed the Channel::Guild variant to reduce the enum size.

[arqunis]

Boxed the Channel::Guild variant to reduce the enum size.

This is a breaking change. You'll have to either remove this change, or rebase the PR to the next branch.

[arqunis]

It is fine otherwise.

[kangalio]

Boxing has already been done in #2205 so it can be safely removed from this PR

[BrettMayson]

This seems to have broken Interaction::MessageComponent. I am getting this error when clicking on a button on a message, the handler is not ran.

WARN run:recv_event:handle_event{event=Err(Json(Error("event InteractionCreate: missing field `available_tags`", line: 0, column: 0)))}: serenity::gateway::shard: [Shard [0, 1]] Unhandled error: Json(Error("event InteractionCreate: missing field `available_tags`", line: 0, column: 0))

[arqunis]

This seems to have broken Interaction::MessageComponent. I am getting this error when clicking on a button on a message, the handler is not ran.

WARN run:recv_event:handle_event{event=Err(Json(Error("event InteractionCreate: missing field `available_tags`", line: 0, column: 0)))}: serenity::gateway::shard: [Shard [0, 1]] Unhandled error: Json(Error("event InteractionCreate: missing field `available_tags`", line: 0, column: 0))

This should be fixed with fc9c420