Additional eligibility checks on ProductVariant in Cart/OrderLine

[chladog]

Is your feature request related to a problem? Please describe.
Currently there are no eligibility checks when adding a ProductVariant to Order (e.g. is Product enabled? isn't Product deleted? is ProductVariant enabled? isn't ProductVariant deleted?). This can result in customer ordering invalid ProductVariant when there is some bug, or cached page, or via manual mutation query by exploiters.

Describe the solution you'd like
Check validity of such action (adding ProductVariant to Order). As well as there should be additional check when transitioning Order to next state from AddingItems that all OrderLines have eligible ProductVariants (incl. enough of stock).