Start and end a session when player enters and exits game screen

[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:
In the Firebase interactor create a...

• field that keeps track of the current user
• field that keeps track of the current session
• method that creates a new session with the user's id and a start timestamp, and adds the session to the user's list of sessions
• method that ends the current session with an end timestamp

For the entering and exiting the game screen:

• When a user enters the game screen, a new session should be created in the Firestore and it should become the current session
• When a user exits the game screen, the current session should end (be given an end timestamp)
• When a user exits the game screen, there should no longer be a current session

Backend brainstorm for reference on schemas.

[nicolesorial]

Manually testing, ran into the following:

For all pairing: (tested with 2, 4, 8 pairs)

• Sessions are tracked in their collection, but not added to the sessions under the user collection
• Same thing for seen pairs

Other:
Users assigned to selecting are still getting pairing -- couldn't test selecting

EDIT: Fixed the game mode bug, and the same issues for pairing exist for selecting