Auto select element types on file import - when possible

The element & folder type selection can be auto filled in some cases:

1. if there is a unique choice, use that choice, and disable the select
2. if there is a folder element type (litterally an elemnt type with folder slug) use that by default for folder type, but still allow selection
3. if there is a page element type, use that by default for element type

Part of #210 (closed)