Shared sync method setting for supplier is no longer respected

[robwa]

It seems (as far as I can see) that the setting Supplier.shared_sync_method is no longer respected when syncing articles with a remote source.

[lentschi]

You're right, @robwa, thanks for raising this issue.
While Supplier.shared_sync_method was still partially supported ('import' does lead to there being an import header in the articles index of the respective supplier), the field isn't handled in the sync_table.

I've started fixing this in #1209, but then remembered why I hadn't implemented this in the first place: I wondered, if we shouldn't add a fourth option availabilty_from_remote.
That is actually how it works now (before the PR): The new sync interface supports availability too. (I thought that made sense - sometimes the supplier or whoever is sharing the supplier resource might want to determine, whether new articles are available.)

[lentschi]

I wondered, if we shouldn't add a fourth option availabilty_from_remote.

I just figured this is always something we can deliver later and finished implementing the PR without it. (After all the sharedlists didn't even have an availability column in its db, so this isn't something that's going to be missed from the start.)