Change the minimum number of players for the game session

[ivellios]

Background

Currently, we have a minimum number of players for the game hardcoded in our backend and frontend to check different logic. The value now is set to 2 players.

Feature improvement

We would like to change this value to be set in the configuration or at least const file, so it can be changed to fit the needs of the community.

Technical details

https://github.com/dnd-al-krk/portal_backend/blob/master/games/models.py#L186 is one of the places where the value is hardcoded. It is also in some places of the frontend code. Should be identified and replaced.