Use select menu for player selection, adding new players

[cheshire137]

Instead of a text field where the user can type a new player name, let's use a select menu. The menu can start with just 'Add a new player' as an option. As the user adds players to their compositions, we can prefill that select menu with every player they've ever created.

As @rewinfrey points out in #57, this will let us simplify the logic in the CompositionSaver model because we can extract player-creation logic from composition-saving logic.