allow playable game width and height to be null

[laurenpudz]

allow width and height of playable game to be null, but only when the embed id is null

• gonna add some validation on the django admin site as well as the model
• will add frontend validation before attempting to render game
• will add explanation in docs

[github-actions]

Branch issue-143-allow_playable_game_width_and_height_to_be_null created!