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
I see now from your comment on the PR that you shifted this check to where the function is being called.

If you have the built-in assumption that this will never be called unless ``SHIFT_CUSTOM_BADGES`` is true, then I strongly strongly strongly recommend putting

```python
assert SHIFT_CUSTOM_BADGES
```

at the top of this functon alongside the other assertions.