#2035 - Centralize definitions of how thumb/resize filenames are generated.

- added constants $album_image_name and $extention_changes to Item_Model.
- added _convert_name_by_size() and _get_mime_type_by_size() to Item_Model to process them.
- added thumb_mime_type() and resize_mime_type() to Item_Model.
- added _find_by_path() and made it a new way to constuct Item_Model.
- stripped item::find_by_path(), as it was revised and moved to Item_Model.
- revised data_rest to use new functions.
- revised file_proxy to use new functions.
- recast item::find_by_path unit tests as item model unit tests.
- removed find_by_path tests for png (now same as jpg case).