Add a form to join an existing game (by 4-letter code) from each game page

Add a form to join an existing game (by 4-letter code) from each game page

We already had the ability to host a new game at these pages, but a
user may land here on want to join an existing game. So give them that
form to be able to do that. (Saves them having to know that they could
go to / to do that, and saves them the work of doing that.)