Disable creation of batchable source-enabled backflow subscriptions

[premun]

Context

In #3378, we added a support for batchable subscriptions.
These only make sense in forward-flow scenarios though. For backflow, it would have to mean there's a subscription to multiple VMRs. This does not make much sense but the tooling (darc, the API) still technically allow creation of the subscriptions. We should forbid this.

Goal

• Make darc error out when a user tries to add/update a subscription and make it batchable + source enabled + backflow (meaning the source directory is set).
• Make the API return 400 when the same happens and someone tries to create a batchable source enabled backflow subscription.

[adamzip]

On top of forbidding the creation of a source-enabled batched subscription, would it make sense to disallow repos from having more than one source-enabled subscription? (this implies subscription to multiple VMRs too, right?)

[premun]

Done, closing