feat(forum): add support for [game=X set=Y] shortcodes

[wescopeland]

This PR adds support for game URLs with set parameters in the forum.

On submit, these are converted to [game={backingGameId}].

[Jamiras]

Should editing a post convert the [game=subsetgameid] back into [game=gameid set=subsetid]?

[wescopeland]

I think we should obscure [game=X set=Y] as an implementation detail and instead reverse [game={backingGameId}] to route('game.show', ['game' => $game, 'set' => $set]. In other words, show the user the absolute URL when they go to edit the post.

This is sufficiently complicated enough that I'm going to merge this and tackle the reversal logic in a subsequent PR.