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