Add ability to mark levels as optional

[bluebear94]

Some games contain levels that aren’t meant to be solved:

1. Natural Numbers Game: Power World, Level 10: Fermat’s Last Theorem
2. Real Analysis, The Game: Lecture 19, Level 4: Conditional Convergence Theorem

It would be nice for these games to have a way to mark such levels as optional, since otherwise, the only way to advance without actually solving them is to switch your game rule setting to relaxed or none. These levels could also show as a different color from other levels.

[joneugster]

Thanks for the suggestion. There is already a feature-request about this from a while ago: #217

I would love to find the time to implement this at some point!