Fix badge renumbering

Completely rewrites the badge renumbering functions.

The primary goal was to make badge auto-numbering work when badge shifting was turned off, as it was assigning duplicate badges in some circumstances.

Secondary goals were:
- making the function easier to read
- removing shift_custom_badges check from the shifting function and instead preventing the function from being called unless shift_custom_badges is true
- automatically renumbering badges back into their range, even if some badges of the same type are out-of-range
- keeping the logic about "which badge is actually considered 'next'" in the next_badge_num function, instead of tacking on adjustments after calling it
This is still in there, it's just not showing here. I essentially replaced it with a check that makes sure it's not in preassigned_badge_types AND ALSO not checked in, since I couldn't see where checked-in attendees were prevented from being assigned badge num 0.