feat: Add ability to make games publicly joinable

[touficbatache]

Add option to let users make their game:

• private (only accessible by game code)
• public (list of publicly joinable games in the join screen) - add warning when choosing if streamer mode on

Linked to touficbatache/SecretHitlerFirebase#6

[touficbatache]

Maybe a chat feature would then be needed? #20

[touficbatache]

Make public games watchable: games that are public can be joined by other people only for the purpose of watching. A game could be made public halfway through, shouldn't be a problem: once a public game status isn't "waiting" or is full, it becomes only watchable and not joinable.