Only care about the custom field's converted name when updating the custom field itself

[snipe]

Related to #11404, this PR switches from us using the ->convertUnicodeDbSlug() modifier in most places (except when saving the custom field). The recent transliteration issues are a real pain in the bunghole, and at the end of the day, we shouldn't care if the custom field db_column is "correct", just that it agrees with whatever is in on the assets table. When a custom field is updated, update that db_column value in the custom_fields table, and then just use the field's db_column value for things moving forward for displaying/editing assets, etc.

[uberbrady]

This will absolutely be much better. Once we’ve vetted this and it’s working well-enough in the wild, maybe we can remove the warning when the fieldnames don’t line up perfectly right? Still, baby steps - let’s get there first. Nice work!