Prevent mislinking of data during import

[ifokkema]

Describe the bug
Individuals, Screenings, and sometimes even Variants on Genome (VOG) with low IDs often have data associated with them that have been uploaded much more recently and is actually unrelated. The cause for this could be missing records in an import or mistyped IDs. For instance, if Screenings in an import file have Individual IDs 1-10 configured but these Individuals are missing from the import file, the Screenings are linked to Individuals 1-10 in the database.

Expected behavior
LOVD should either predict what the user is trying to do or ask for confirmation. E.g., see that the data that is being linked to has been uploaded a long time already or is unrelated somehow, and block the import. Or, ask the user for confirmation as some of the data referenced from the import file can not be found in the file itself.