Add UX for games that modified the disc

[mattgodbolt]

As of 5b9135f discs are read/write, but lose contents on window close. Would be ace to do something like one of:

• hashing contents and making a local disc image named by original disc hash, save by that, and offer to load the modified disc on load.
• popping up a message that notes the disc has changed, and offers a way to make a local image
• Dialog box (ugh) saying "is this ok?"

I'm error towards the first item, or a combination of first and second. Adding a UI to look at the locally-saved disc images would be good too.

[sassquad]

You would want to ensure that you an steer away from browser default alert messages. Chrome is discouraging use of these anyway, as they're typically used by malicious sites.

Custom modals are probably the way forward.