TO DO
- auto-unlock
- check lock status from the series title page (save unlocked chapter status to) ?? Hard to find a way for this without making excessive queries
- prevent adding multiple event listeners
- debouncing
- weak maps
- spinner to be implemented for the button
- disable button when no chapters are selected
- checkboxes not cleared when error
- console output as completed after the unlock process
- start unlocking from the lowest chapter