Split custom_badges_really_ordered

Fixes #545 by splitting custom_badges_really_ordered into add_custom_badges and shift_custom_badges.

Keep in mind that the logic has flipped - shift_ and add_ need to be True by default.
So you're saying that if they have 200 numbered badges and delete badge number 17, they explicitly do NOT want the system to renumber the other badges to fill the hole?  I mean... okay.  I'm not sure I see the sense in that, but Anthrocon does registration really well from what I've seen, so I generally give them the benefit of the doubt and assume they know what they're doing and have good reasons for this kind of stuff.

(To make that workflow work, they'll also need a page which shows them which badge numbers are not taken despite being between two other badges numbers which ARE taken.  I mean, I assume they'd want that, but that can just be part of their own plugin.)

So there are two sensible ways we could tackle this:
1. Have two settings; ``SHIFT_BADGES`` and ``EDIT_BADGES``.  We'll use the first one like you're already using it, and use the second one for literally only this one form field.
2. Just use ``BEFORE_PRINTED_BADGE_DEADLINE`` just like we do on all of the attendee-facing forms.  The reason we don't is because there's usually two deadlines, the attendee-facing one and then the "real" one where Paul says "ok I submitted the order".  We rarely use this difference to make printed badge adjustments, and in the worst case we could just update the database.