#43 Add "beginner lesson" support

[Alf Mikula]

Part of the main mission of PromoteGo.org is to bring new players to the game. Let's make it easy for people to offer "beginner lessons" by providing a checkbox in the "Offer Game" form to indicate that users may accept as a beginner lesson.

Games accepted as beginner lessons should remain open for multiple players to accept. Let's set a limit of 5 on the number of players allowed to accept a single beginner lesson for now.