#2001 - Make filename sanitizing more consistent.

- legal_file - added sanitize_filname() to sanitize photo/movie filenames.
- admin_watermarks - revised add() to use new function.
- item model - added _process_data_file_info() to validate the data file, get its metadata, and sanitize the item name.
- item model - revised save() for new items to use _process_data_file_info *before* the slug is checked.
- item model - revised save() for updated items to use _process_data_file_info.
- item model - revised save() for updated items to sanitize name if changed.
- uploader - removed call to smash_extensions (item model does this when it calls sanitize_filename).
- Legal_File_Helper_Test - added unit tests for sanitize_filename.
- Item_Model_Test - revised existing unit tests based on changes.
- Item_Model_Test - added new unit tests for names with legal but incorrect extensions.
- Averted take over by HAL with fix #2001...