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
Was this removed because Anthrocon no longer has ``PREASSIGNED_BADGE_TYPES``?  I mean, I know that all badge types are preassigned for Anthrocon folks, but I assumed that just meant that they'd include all of those types in the ``PREASSIGNED_BADGE_TYPES`` setting.