Save completed problems

[blobotic]

We need an option to mark a problem as completed and to save it (localstorage?) to be accessed later.

Also under [Stats], display a list of all completed problems. (Potentially unnecessary if #7 works; if so, just display a count, e.g. Completed: 37/40)

(Potential uses: fixing random, changing appearance of problems)

[blobotic]

This probably uses a structure of like

localstorage = {
  "problems": [{
    "name": "NACLO",
    "yrs": [{
      "yr": "2022",
      "ps": [{"number": "A", "completed": true/false, "answers": {"id1": "content"}}]
    }]
  }
}]