Start and end a round when player selects to play again

[emaela]

Entering the game screen should trigger a new session to be created, and exiting the game screen should trigger the current session to end.

Specs:

• When a player first enters the game screen, a new round should be created and added to the current session with a start timestamp and list of word pairs.
• When a player presses 'done', the current round should be ended with an end timestamp and list of matches.
• When a player presses 'play again', a new round should be created and added to the current session with a start timestamp and list of word pairs.
• When a player presses 'end session', the current round should be ended with an end timestamp and list of matches.

Backend brainstorm for reference on schemas.