resole issue #19: make sure game keeps at least one owner

app/main/routes_player.py

@@ -43,6 +43,10 @@ def add_player(game_name):
 def remove_player(game_name, username):
     game = get_game_if_owner(game_name)
     user = User.query.filter(and_(User.name == username, User.games.contains(game))).first_or_404()
+    if user.role_in_game(game) == Role.owner:
+        if len([player for player in game.players if player.role == Role.owner]) < 2:
+            flash("Can't remove only owner from game")
+            return redirect(url_for('main.game_dashboard', game_name=game.name))
     game.users.remove(user)
     if not user.last_login:
         db.session.delete(user)