Create link to join a game

[joffrey-bion]

It would be nice for game owners to share a link that allows other players to join a game directly without going through the game browser.

The link would bring them to a page similar to the current home (because they need to pick their username), but with a mention that they're about to join a specific game (with the game's name). When they validate, they would directly go to the lobby of the game.

We could also have a button to just connect as usual without joining.