BUTTON Logic -- activating START game button

Start game button should not be active until AT LEAST 1 salesperson and 1 buyer role are assigned in the roles categories. if they both become sellers, and no one is a buyer, then the start game button should deactivate (and be grayed out),

Start at the start game button and the roles-assignment logic. Verify the button is disabled and visually grayed out until at least one salesperson and one buyer are assigned, including when all assigned roles become sellers; done means these states behave consistently.

Written by the indexing model from the issue text.