Save load progress simple. Closes #102

[MoweMax]

This PR contain changes for #102. Current progress (and last 5 bingo sheets with progress [older saves will be overwritten]) are automatically stored in localStorage and will be auto-restored (load) when url with same settings (SEED, DIFFICULTY, VERSION) is opened. Also added button to generate url with current progress to store progress indefinitely (to continue later or share with others). Progress in url will be autoupdated upon changes to the sheet.

Demo:

https://user-images.githubusercontent.com/16069758/114303719-3b186500-9ad8-11eb-9e94-9ba32d2b0c83.mov

[rybak]

I've reviewed most of the JS changes. I've glanced over HTML, but haven't looked at CSS changes yet. I'll continue the review later.