Add "Invite to game session" action

{"ops":[{"insert":"Quite a simple one - yeah, player can just copy URL from the browser's navigation bar, but that's not intuitive at all.\nSo to make things simpler for the user - new action button will be added to the top of the game session page - clicking on which will open \"share\" modal, which for now will only contain a link, and \"copy\" button.\nIn the future, with addition of social feature - same modal can be extended with \"Invite Friends\" functionality.\n"}]}