Allow filtering certain dates from puzzle set date picker ui
Right now, puzzle sets that download puzzles using a date picker allow the user to select any date. Most online puzzles have a start date, where no puzzles exist before that. Some puzzles also have no puzzles available on certain dates, where some of those are predictable (Telegraaf themed crossword, only on Saturdays) and some are not (The Guardian, missing dates). To avoid unclear errors, it would be preferred that this is addressed in the user interface. For example: Dates that are known to have no puzzle are disabled in the picker.