Filter features on import

[garrettjstevens]

If a user only wants to edit certain types of features, be able to set a filter so only those features are imported.

Low-priority issue for now since 1) this can be done outside of Apollo by filtering the GFF3 before import and 2) the user would likely have to merge the resulting GFF3 with their original, and we don't have a good way of suggesting how to do that.