fix: bugs with changing pet variants

[perappu]

Fixes a few bugs with changing pet variants (and adds a little bit of consistency polish while I was already there).

When changing a pet's variant, the drop_id on its related PetDrop falls out of sync. This can cause a variety of rather insidious and vague errors (usually something to do with a null value when looking for drops_available), especially when trying to mass collect drops. This fix ensures it stays in sync with its new pet_id.

I have included a console command php artisan fix-pet-drop-ids which will fix existing inconsistent PetDrops. I also included fixes to the debitStack and editVariant functions that will prevent the creation of orphaned rows in the pet_drops table (the command will also delete any existing orphaned rows). It's been thoroughly tested on both my local and two separate live sites, so I'm pretty sure it doesn't accidentally explode anything critical.

This PR also includes a fix for when trying to set a pet back to its "default", since it wasn't yet updated for the pet parent_id change.

A warning is now also displayed to both users and staff when you're about to change the variant on a pet that has pending drops to collect:

Let me know if you want me to just disable users from being able to change variants entirely when there's pending drops, might make it a little more oh-no-I-didn't-read-regret-proof. :P

Thanks again!