Data Tables can now import from JSON, and this was taking priority over the Paper2D importers. There were a few issues here:
1) FAssetTools::ImportAssets wasn't respecting the priority of the import factories, and was just testing them in the order they were found.
a) UCSVImportFactory needed to be given a lower than normal priority so it gets tested last as it will perform a catch-all match.
2) FAssetTools::ImportAssets will default to the first factory it finds, and then use FactoryCanImport to try and find a better match, however FactoryCanImport returns false by default, so...
a) UPaperSpriteSheetImportFactory needed to implement FactoryCanImport to avoid sprite sheets being imported as Data Tables.
b) UCSVImportFactory needed to implement a catch-all FactoryCanImport so it can be used when no other factory is a better match.
#codereview Andrew.Rodham, Michael.Noland
[CL 2608928 by Jamie Dale in Main branch]
b97258a833