Enable basic playing of turns

This pull request is mostly concerned with removing the mocking to allow playing of active turns and saving of game state after a turn has been played.

Note that a turn will fail for many reasons, including:
- you are not logged in as the attacker player
- the attack is not valid
- the current view of the game is not up to date

To play a turn for an active game that already has swing values set for both players:
- activate it from the list of active games at overview.html
- view it at play_turn.html
- select the attacking and defending dice
- submit the attack