Display warning when approaching save limit

[g-liu]

See https://iafisher.com/projects/cities/faqs

You cannot save a game with more than 5,000 cities (7,500 for the world quiz). However, you can continue adding cities to your map; you just won't be able to save your results permanently and get a shareable link.

We need to address this with a warning (say around 7000 cities?) and then disable the Finish button with an explanation.

If user does exceed the limit and still wishes to save, we should provide them with the capability to remove cities from the guessed list. This would entail writing to the stats provider and not just reading 😲