Disallow importing partial datasets from the server

[atomrab]

I made a local collection and imported changes from the Chersonesos authority I made a month or so ago. This was the only authority I imported. I then added a new period and made some minor tweaks to the editorial note in another, and submitted the changed version as a patch.

In the patch change summary, I saw not only those changes but the deletion of all of the derived-from relations I had included in the original authority. These deletions persisted even when I added the authorities from which those periods were derived. See below (also, oddly, in that record only some of the linked narrower definitions appear as links, though they're all linked when one looks at the authority in browse or edit view).

Adding new periods to existing authorities that have derived-from relationships seems like it will be an increasingly common use-case. We don't want to lead people to accidentally erase those existing relationships.