fix: sync lowInventoryThreshold number between variants and child options

[kieckhafer]

Resolves #4358
Impact: minor
Type: fix

Issue

Low inventory threshold numbers are stored only on a product variant, not on it's options. We have login in our code to attach the variants number to each option, however even with this logic, in the database, the lowInventoryThreshold is always set to 0 on the the option.

Solution

We are keeping the logic the same, and how we read the numbers the same, but we are now syncing the lowInventoryThreshold number to each option, when it's changed on it's parent variant. This may create easier use in the future, but should not create any real changes now, as option.lowInventoryThreshold isn't used anywhere. The only change seen should be the number inside the database.

Breaking changes

None

Testing

1. Add a product with variants and options
2. Add a "warn at" number to the product variant
3. Open 3T and see the same number is saved as the lowInventoryThreshold number on that variant, as well as all the option children of said variant.

[willopez]

LGTM