Share checklist of problems solved

[fishy15]

Extend what was implemented for #6. It would be useful if users could share the problems they have solved. This can be done by exporting some JSON that includes which problems are solved and storing it in base64, which can be viewed by some URL. I'm not sure how OI Checklists does it, but they don't seem to be doing it this way (probably something stored on the server).

[thecodingwizard]

A note about sharing via URL -- to my understanding there is a maximum length that URLs can have. It might be better to store the data in a backend database (ie. firebase or something).