Add a "allow friends" option, in addition to game passwords

[samuelweiler]

Is your feature request related to a problem? Please describe.
I'd like to let my /friends join games I create, whether or not they know the password for the current game.

Describe the solution you'd like
Could we have a game option that, for games with a password set, also allows anyone on my friend list to join? I presume this would add a checkbox to the new game dialog near/after the password field that says "Also allow friends", with a help link to the /friend definition in chat-commands.md