Regenerate allocations

[dlhck]

Migrated from #661

Is your feature request related to a problem? Please describe.
After few weeks of implementation of stock movement feature we have quite a few product variants that because of an transition error, sometimes for unknown reason have unexpected stock allocation values - e.g. we have product variants that have negative allocations, or had some orders allocated multiple times.
The main issue to address here is ofc refinement of our project-specific order process and error handling to keep track of right allocations as much as possible, though I believe there will be edge cases/unpredictable errors that can result in wrong allocations that don't match to open orders. Currently when this happens there is no easy way how to get or update the current allocations.

Describe the solution you'd like
I propose a new function + button in AdminUI regenerate allocations, that when run would cycle through open orders and adjust allocations of all product variants that have invalid values. (and reset all allocations for product variants that are not in any of open order to 0).