Move generate-title-if-missing code to Model_Item.

- add code to Model_Item::create(), similar to how generate-slug-if-missing.
- remove same code from server_add, uploader, and add albums form.
- add trim() to Item::convert_filename_to_title() to avoid titles like " ".
- revise Item_Model_Test to reflect changes.
- add to Item_Test to reflect changes.