Abstract out the concept of a JoinableGame

For example Rock paper scissors has the concept of people choosing to join the game.

We're going to create a Heist game as well, so that we can see what's consistent, extract it, and test that portion separate from the rest of the join-able games.

Start by locating the Rock paper scissors implementation and identifying the existing logic for players choosing to join. Compare that behavior with the planned Heist game, extract the shared JoinableGame concept, and add separate tests for the shared joining behavior.

Written by the indexing model from the issue text.