StockLevel not created when adding new ProductVariant via Admin UI

[michaelbromley]

Describe the bug
When adding new variants via the "manage variants" screen in the admin ui, the variants are created but without an associated StockLevel.

There should be a way to specify a stock level in a given stock location during the process of creating the variant.

Environment (please complete the following information):

• @vendure/core version: 2.0.4
• Nodejs version: any
• Database (mysql/postgres etc): any

[dlhck]

Should be done together with #2903